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/test87

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -8,7 +9,6 @@ globbing
 FAILURE
 </keywords>
 </info>
-#
 # Server-side
 <reply>
 <data1 crlf="headers">
@@ -27,7 +27,6 @@ two is nice too
 </data2>
 
 </reply>
-#
 # Client-side
 <client>
 <server>
@@ -44,10 +43,8 @@ urlglob with out of range -o #[num] usage
 </command>
 </client>
 
-#
 # Verify data after the test has been "shot". Note that the command line
 # writes both responses into the same filename so only the second survives
-#
 <verify>
 <file name="%LOGDIR/dumpit%TESTNUMBER-#2.dump" crlf="headers">
 HTTP/1.1 200 OK