tests/data: more XML-compliance via %LT and %GT macros in email addresses

Reduce number of files failing `xmllint --format` from 133 to 57 (-76)
(3% of 1894), by replacing `<` and `>` with new macro `%LT` and `%GT`,
in most places, which is in email addresses (192 lines).

Follow-up to a9ec2a676c #19491

Closes #19470

tests/testutil.pm

@@ -156,6 +156,8 @@ sub subbase64 {
     $$thing =~ s/%SP/ /g;    # space
     $$thing =~ s/%TAB/\t/g;  # horizontal tab
     $$thing =~ s/%CR/\r/g;   # carriage return aka \r aka 0x0d
+    $$thing =~ s/%LT/</g;
+    $$thing =~ s/%GT/>/g;
 
     # include a file
     $$thing =~ s/%include ([^%]*)%[\n\r]+/includefile($1, 0)/ge;