runtests: improve XML prolog check, enable -w permanently, fix two tests

To really verify the presence of the XML prolog, also in CI.

- move the prolog check from `loadtest` to `checktest`.
  (load did a soft error, silently skipping the test instead of failing)
- runtests: enable `-w` functionality permanently for all test targets,
  drop the option. It has no measurable performance impact.
- test 798, 1665: add XML prolog.
  Follow-up to f0d277cb0e

Follow-up to b5ea0736bb #19946
Follow-up to 904e7ecb66 #19347

Closes #19970

tests/data/test1665

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -5,7 +6,6 @@ HTTP
 </keywords>
 </info>
 
-#
 # Server-side
 <reply>
 <data>
@@ -22,7 +22,6 @@ Funny-head: swsclose
 </data>
 </reply>
 
-#
 # Client-side
 <client>
 <server>
@@ -36,7 +35,6 @@ http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>
 
-#
 # Verify data after the test has been "shot"
 <verify>
 <protocol crlf="headers">

tests/data/test798

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="US-ASCII"?>
 <testcase>
 <info>
 <keywords>
@@ -7,9 +8,8 @@ cookies
 cookiejar
 </keywords>
 </info>
-# Server-side
-#
 
+# Server-side
 <reply>
 <data crlf="headers" nocheck="yes">
 HTTP/1.1 200 OK