runtests: improve XML prolog check, enable -w permanently, fix two tests

To really verify the presence of the XML prolog, also in CI.

- move the prolog check from `loadtest` to `checktest`.
  (load did a soft error, silently skipping the test instead of failing)
- runtests: enable `-w` functionality permanently for all test targets,
  drop the option. It has no measurable performance impact.
- test 798, 1665: add XML prolog.
  Follow-up to f0d277cb0e

Follow-up to b5ea0736bb #19946
Follow-up to 904e7ecb66 #19347

Closes #19970

tests/runner.pm

@@ -1162,7 +1162,7 @@ sub singletest_postcheck {
         }
     }
 
-    if($checktests && checktest("${TESTDIR}/test${testnum}")) {
+    if(checktest("${TESTDIR}/test${testnum}")) {
         logmsg " $testnum: postcheck FAILED: issue(s) found in test data\n";
         return -1;
     }