tests/data: move --libcurl output to external data files

To make the test files XML-compliant, and the expected results
possibly easier to manage by keeping them in `.c` files.

Non-XML-compliant files are down to 36 after this patch.

Also:
- make all macro expansions apply to `%includetext` contents.

Closes #19799

docs/tests/FILEFORMAT.md

@@ -115,7 +115,8 @@ 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:
+text file, and whitespace, special character macros and variables expanded
+after inclusion:
 
     %includetext filename%