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/testutil.pm

@@ -158,6 +158,7 @@ sub subbase64 {
     $$thing =~ s/%CR/\r/g;   # carriage return aka \r aka 0x0d
     $$thing =~ s/%LT/</g;
     $$thing =~ s/%GT/>/g;
+    $$thing =~ s/%AMP/&/g;
 
     # include a file
     $$thing =~ s/%include ([^%]*)%[\n\r]+/includefile($1, 0)/ge;