tests/data: add XML prolog to test files

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 87ba80a6df

Closes #19946

.github/workflows/checksrc.yml

@@ -135,11 +135,7 @@ jobs:
           persist-credentials: false
 
       - name: 'check'
-        run: |
-          {
-            git grep -i -l -E '^<\?xml' -z || true
-            git ls-files 'tests/data/test*' -z || true
-          } | xargs -0 -r xmllint >/dev/null
+        run: git grep -i -l -E '^<\?xml' -z | xargs -0 -r xmllint >/dev/null
 
   miscchecks:
     name: 'misc checks'