tests: speed up builds with single-binary test bundles

Add support for single-block binaries that contain all libtests and
unit tests respectively.

Enable with:
- autotools:

tests: speed up builds with single-binary test bundles

Add support for single-block binaries that contain all libtests and
unit tests respectively.

Enable with:
- autotools: `--enable-test-bundles`
- cmake: `-DCURL_TEST_BUNDLES=ON`

(They are compatible with `--enable-unity` and `-DCMAKE_UNITY_BUILD=ON`
options, for further speed-up.)

Makes libtests and unit tests build _fast_, needing little disk space
even in static mode. Similar to CMake unity mode, but with a custom
script, also supporting autotools builds.

The price is having to deal with symbols/macros colliding between
`lib*.c` and `unit*.c` sources. Maybe with naming conventions or other
solutions this can be improved gradually and reduce the need for manual
intervention by `mk-bundle.mk`. I've included a script that does the bulk
of detecting name collisions.

Also:
- CI: enable test bundles.
- CI: build tests in more jobs.
- lib2305: fix FILE handle leak.
- unit1661: fix memleak found by torture test by releasing the `bufref`
structure in `unit_stop()` that was allocated in `unit_setup()`.
```
test 1661...[bufref unit tests]
Leak detected: memory still allocated: 13 bytes
allocated by /home/runner/work/curl/curl/tests/unit/unit1661.c:70
1661: torture FAILED: function number 1 in test.
```
Ref: https://github.com/curl/curl/actions/runs/10967279334/job/30456745290?pr=14772#step:8:41

Similar test suite builds with autotools default and cmake+bundle+unity:
- GHA/Linux: 33s vs 7s
https://github.com/curl/curl/actions/runs/10705668823/job/29681617374
- GHA/macOS 34s vs 2s
https://github.com/curl/curl/actions/runs/10705668813/job/29681632885
- GHA/FreeBSD: 15m25 vs 6m21 (full workflow time, ~qemu)
https://github.com/curl/curl/actions/runs/10705668811/job/29681607915
- GHA/Cygwin: 9m52 vs 32s
https://github.com/curl/curl/actions/runs/10705668809/job/29681609965
- GHA/MSYS2: 3m52 vs 14s
https://github.com/curl/curl/actions/runs/10705668808/job/29681624295
- GHA/mingw-w64: 5m45 vs 30s
https://github.com/curl/curl/actions/runs/10705668808/job/29681628787

Autotools test suite builds compared between master -> `--enable-test-bundles`:
- GHA/Linux: 33s -> 9s (run tests: 22m23 -> 20m44)
https://github.com/curl/curl/actions/runs/10710030193/job/29695932185
https://github.com/curl/curl/actions/runs/10967831456/job/30458220344
- GHA/macOS: 25s -> 4s (run tests: 2m58 -> 2m24)
https://github.com/curl/curl/actions/runs/10710030195/job/29695938444
https://github.com/curl/curl/actions/runs/10967831452/job/30458225762
- GHA/non-native (FreeBSD): 4m8 -> 3m12 (full workflow time, ~qemu)
https://github.com/curl/curl/actions/runs/10710030198/job/29695928401
https://github.com/curl/curl/actions/runs/10967831458/job/30458212692
- GHA/Cygwin: 9m25 -> 1m9 (run tests: 9m19 -> 3m28)
https://github.com/curl/curl/actions/runs/10710030212/job/29695928213
https://github.com/curl/curl/actions/runs/10967831453/job/30458213268
- GHA/MSYS2: 3m54 -> 32s (run tests: 6m3 -> 3m59)
https://github.com/curl/curl/actions/runs/10710030190/job/29704850591
https://github.com/curl/curl/actions/runs/10967831449/job/30459280005
- GHA/mingw-w64: 5m42 -> 1m5 (run tests: 7m41 -> 5m36)
https://github.com/curl/curl/actions/runs/10710030190/job/29704852058
https://github.com/curl/curl/actions/runs/10967831449/job/30459280862
- Azure MSYS2 mingw64 openssl: 38m55 -> 11m58
https://dev.azure.com/daniel0244/curl/_build/results?buildId=25546&view=logs&j=b58b8c59-0f61-52e9-0f9e-fad562a1e77f&t=0f9230a7-3b10-53ca-9938-700ece377c5e
https://dev.azure.com/daniel0244/curl/_build/results?buildId=25547&view=logs&jobId=39473db1-3945-55d5-deb5-c218fad88dce&j=b58b8c59-0f61-52e9-0f9e-fad562a1e77f&t=0f9230a7-3b10-53ca-9938-700ece377c5e
- Azure Ubuntu default: 2m15 -> 55s (all build)
https://dev.azure.com/daniel0244/curl/_build/results?buildId=25546&view=logs&j=9d58b9ac-e1e6-53b6-f83a-1f9f1d912522&t=a6b38d83-e7cf-5a9b-c762-a178412717b7
https://dev.azure.com/daniel0244/curl/_build/results?buildId=25547&view=logs&jobId=39473db1-3945-55d5-deb5-c218fad88dce&j=9d58b9ac-e1e6-53b6-f83a-1f9f1d912522&t=a6b38d83-e7cf-5a9b-c762-a178412717b7

Cmake test suite builds compared between master -> `-DCURL_TEST_BUNDLES=ON` + unity:
- GHA/Linux: 29s -> 7s (run tests: 4m50 -> 4m57, 20m43 -> 20m45)
https://github.com/curl/curl/actions/runs/10710030193/job/29695941814
https://github.com/curl/curl/actions/runs/10705668823/job/29681622201
- GHA/Linux old: 44s -> 13s (bundle+no unity) (run tests: 5m5 -> 5m6)
https://github.com/curl/curl/actions/runs/10718264094/job/29719794727
https://github.com/curl/curl/actions/runs/10718653175/job/29721009613
- GHA/macOS: 32s -> 2s (run tests: 2m43 -> 2m40)
https://github.com/curl/curl/actions/runs/10710030195/job/29695931956
https://github.com/curl/curl/actions/runs/10705668813/job/29681638937
- GHA/non-native (*BSD): inconclusive (full workflow time, ~qemu)
https://github.com/curl/curl/actions/runs/10710030198
https://github.com/curl/curl/actions/runs/10705668811
- GHA/Cygwin: 3m9 -> 32s
https://github.com/curl/curl/actions/runs/10710030212/job/29695929075
https://github.com/curl/curl/actions/runs/10705668809/job/29681609965
- GHA/MSYS2: 2m24 -> 14s
https://github.com/curl/curl/actions/runs/10710030190/job/29704850996
https://github.com/curl/curl/actions/runs/10705668808/job/29681624295
- GHA/mingw-w64: 3m56 -> 30s (run tests: 4m2 -> 3m52)
https://github.com/curl/curl/actions/runs/10710030190/job/29704852219
https://github.com/curl/curl/actions/runs/10705668808/job/29681631393
- GHA/mingw-w64-old: 7m19 -> 1m44 (run tests: 3m30 -> 2m53)
https://github.com/curl/curl/actions/runs/10710030190/job/29704849763
https://github.com/curl/curl/actions/runs/10705668808/job/29681622329
- GHA/MSVC: 3m22 -> 13s (run tests: 9m43 -> 4m22)
https://github.com/curl/curl/actions/runs/10710030190/job/29704850411
https://github.com/curl/curl/actions/runs/10705668808/job/29681623313
- AppVeyor CI MSVC 2008: 4m3 -> 45s (full build)
- AppVeyor CI MSVC 2010: 2m56 -> 1m8 (full build)
- AppVeyor CI MSVC 2022: 10m19 -> 2m23 (full build)
https://ci.appveyor.com/project/curlorg/curl/builds/50538455
https://ci.appveyor.com/project/curlorg/curl/builds/50536558
- AppVeyor CI total build time: 10m30 (master) -> 6m48 (unity) -> 4m5 (bundle) -> 3m24 (bundle+unity) -> 5m7 (bundle+unity+all jobs building tests)

Closes #14772

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