tests/data: add %includetext, dedupe XML payloads into external file

To reduce duplication and to avoid keeping XML-like markup within XML
markup (`test*`), that was tripping `xmllint`.

Ref: #19470

Closes #19504

docs/tests/FILEFORMAT.md

@@ -106,6 +106,11 @@ the include instruction:
 
     %include filename%
 
+Or, a variant of the above where the file is loaded as a newline-agnostic
+text file, and `%CR`, `%SP`, `%TAB` macros are expanded after inclusion:
+
+    %includetext filename%
+
 ## Conditional lines
 
 Lines in the test file can be made to appear conditionally on a specific