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
Unify the case, punctuation and name of test case keywords so they can
be more easily selected or skipped when desired. Add a few keywords that
were missing. Fix a couple of typos in test names.
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
