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/test56

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -27,15 +28,11 @@ http
 HTTP POST with *HUGE* request and chunked transfer-encoding
 </name>
 <stdin>
-#
 data "we post this"
-#
 # enforce chunky post!
 header "Transfer-Encoding: chunked"
-#
 # Set the user-agent using a short-option:
 -A "%repeat[99999 x a]%"
-#
 </stdin>
 <command>
 -K - %HOSTIP:%HTTPPORT/that.site.com/%TESTNUMBER