tests: update some HTTP/2 over HTTPS tests

- improve descriptions
- require http/2, not h2c, since they are done over HTTPS

Closes #15317

tests: check http/2 and http/3 server responsiveness

Check responsiveness of http/3 server when running.

Also, a test case with http/2 or http/3 server requirement
now implicitl

tests: check http/2 and http/3 server responsiveness

Check responsiveness of http/3 server when running.

Also, a test case with http/2 or http/3 server requirement
now implicitly drags in a 'http' server and we need no longer
mention that in testdata.

Closes #15058

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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: add tests for HTTP/2 and HTTP/3 to verify the header API

Test 2403 and 2503 check "header_json" output and therefore use of
header-api

Closes #10495