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/test856

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -7,7 +8,6 @@ FAILURE
 </keywords>
 </info>
 
-#
 # Server-side
 <reply>
 <servercmd>
@@ -15,7 +15,6 @@ REPLY PASS -ERR Login failure
 </servercmd>
 </reply>
 
-#
 # Client-side
 <client>
 <server>
@@ -29,14 +28,12 @@ pop3://%HOSTIP:%POP3PORT/%TESTNUMBER -u user:wrong
 </command>
 </client>
 
-#
 # Verify data after the test has been "shot"
 <verify>
 # 67 - CURLE_LOGIN_DENIED
 <errorcode>
 67
 </errorcode>
-#
 # The multi interface considers a broken "CONNECT" as a prematurely broken
 # transfer and such a connection will not get a "QUIT"
 <protocol crlf="yes">