tests/data: mark non-XML-compliant files as such, xmllint the rest in CI

tests/data: mark non-XML-compliant files as such, xmllint the rest in CI

There are 58 non-compliant files. Mark them with the `notxml` keyword.
Also include the compliant ones in the GHA/checksrc xmllint CI job.

Also:
- delete XML prolog from the 3 test data files that had them.
- FILEFORMAT.md: document the `notxml` keyword.
- FILEFORMAT.md: fix a typo.

Follow-up to de49cc89abc917cb4f273ebea8c6fb584d097de2 #19470
Follow-up to f3095f0dbd7e842d4a72c0300ba4817a755c74f5 #19528
Follow-up to 87ba80a6df1dfd7ceaaa52352c9f23afff0ed513

Closes #19595