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/test443

@@ -1,12 +1,11 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 # perl:
-#
 #for(1 .. 20) {
 #    print join("\t",
 #               "attack.invalid", "TRUE", "/", "FALSE", "0",
 #               "huge-$_", ('a' x 500)."-$_")."\n";
 #}
-#
 <info>
 <keywords>
 HTTP
@@ -15,7 +14,6 @@ cookies
 </keywords>
 </info>
 
-#
 # Server-side
 <reply>
 <data>
@@ -28,7 +26,6 @@ Content-Length: 6
 </data>
 </reply>
 
-#
 # Client-side
 <client>
 <server>
@@ -67,7 +64,6 @@ cookies
 </features>
 </client>
 
-#
 # Verify data after the test has been "shot"
 <verify>
 <protocol crlf="headers">