To formalize they are now XML-compliant (with some asterisks.)
Also to help syntax highlighters work on them to make their content more
readable.
Also:
- Delete empty comment decorations.
- GHA/checksrc: simplify XML check.
- runtests: fail to load test data with XML prolog missing.
Follow-up to bfe6eb1c06#19927
Follow-up to 87ba80a6dfCloses#19946
To make them explicit, visible, avoid being accidentally trimmed.
Also prevents Git warnings, e.g. on `git am`.
Also:
- runtests: add support for `%spc%` and `%tab%` macros.
- test59: delete non-significant line-ending space.
- spacecheck.pl: drop line-ending whitespace exception for tests.
Closes#19300
