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

tests/data/test4

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -7,7 +8,6 @@ HTTP added headers
 HTTP replaced headers
 </keywords>
 </info>
-#
 # Server-side
 <reply>
 <data>
@@ -20,7 +20,6 @@ Funny-head: yesyes
 </data>
 </reply>
 
-#
 # Client-side
 <client>
 <server>
@@ -44,7 +43,6 @@ http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>
 
-#
 # Verify data after the test has been "shot"
 <verify>
 <protocol crlf="yes">