compiletest: test suites handle normalizations differently and inconsistently

[jieyouxu]

Noticed in #128929 (comment).

Some test suites use normalize_output when error format is JSON, some test suites do a bit of their own normalization with custom rules, and what we end up with is likely some duplicate logic and subtle inconsistencies. We should probably have e.g. paths normalized consistently across test suites, and document the suite-specific normalizations.