tests: add %AMP macro, use it in two tests

To allow replacing `&` characters in `tests/data/test*` files for
XML-compliance.

Also:
- document `%GT`, `%LT`
  Follow-up to de49cc89ab #19470

Closes #19824

tests/data/test45

@@ -4,7 +4,6 @@
 HTTP
 HTTP GET
 followlocation
-notxml
 </keywords>
 </info>
 # Server-side
@@ -13,7 +12,7 @@ notxml
 HTTP/1.1 301 This is a weirdo text message swsclose
 Date: Tue, 09 Nov 2010 14:49:00 GMT
 Server: test-server/fake
-Location: data.cgi?moo=http://&/%TESTNUMBER0002
+Location: data.cgi?moo=http://%AMP/%TESTNUMBER0002
 Connection: close
 
 This server reply is for testing a simple Location: following
@@ -32,7 +31,7 @@ If this is received, the location following worked
 HTTP/1.1 301 This is a weirdo text message swsclose
 Date: Tue, 09 Nov 2010 14:49:00 GMT
 Server: test-server/fake
-Location: data.cgi?moo=http://&/%TESTNUMBER0002
+Location: data.cgi?moo=http://%AMP/%TESTNUMBER0002
 Connection: close
 
 HTTP/1.1 200 Followed here fine swsclose
@@ -66,7 +65,7 @@ Host: %HOSTIP:%HTTPPORT
 User-Agent: curl/%VERSION
 Accept: */*
 
-GET /want/data.cgi?moo=http://&/%TESTNUMBER0002 HTTP/1.1
+GET /want/data.cgi?moo=http://%AMP/%TESTNUMBER0002 HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 User-Agent: curl/%VERSION
 Accept: */*