tests: replace hard-coded `/dev/null` with variable

- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
-

tests: replace hard-coded `/dev/null` with variable

- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
- add `%DEV_NULL` variable and use it in tests.

Fixes `The system cannot find the path specified.` messages seen when
running `runtests.pl` with native Windows Perl.

Also adjust code to not break mcedit syntax highlighting.

Cherry-picked from #14949
Closes #15098

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tests: remove leading spaces from some tags

The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so

tests: remove leading spaces from some tags

The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so
widespread in testcases is probably that they have been copy and pasted.

Hence, fixing them all now might curb this practice from now on.

Closes #12028

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tests: use %TESTNUMBER instead of fixed number

This makes the tests easier to copy and relocate to other test numbers
without having to update content.

Closes #6738

runtests: provide curl's version string as %VERSION for tests

... so that we can check HTTP requests for User-Agent: curl/%VERSION

Update 600+ test cases accordingly.

Close

runtests: provide curl's version string as %VERSION for tests

... so that we can check HTTP requests for User-Agent: curl/%VERSION

Update 600+ test cases accordingly.

Closes #6037

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test3015: verify stdout "as text"

Follow-up from 0c1e767e83e to please win32 tests

Closes #5962

tool_writeout: add new writeout variable, %{num_headers}

This variable gives the number of headers.

Closes #5947