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 de49cc89ab #19470
Follow-up to f3095f0dbd #19528
Follow-up to 87ba80a6df

Closes #19595

.github/workflows/checksrc.yml

@@ -128,7 +128,11 @@ jobs:
           persist-credentials: false
 
       - name: 'check'
-        run: git grep -z -i -l -E '^<\?xml' | xargs -0 -r xmllint >/dev/null
+        run: |
+          {
+            git grep -z -i -l -E '^<\?xml' || true
+            git grep -z -L -F 'notxml' 'tests/data/test*' || true
+          } | xargs -0 -r xmllint >/dev/null
 
   miscchecks:
     name: 'misc checks'