Remove proc_macro from the tidy whitelist again

PR #38842 has exposed that we were missing the src/test/compile-fail-fulldeps
directory in the search for feature gate tests. Because the detection didn't
work despite the effort to name the test appropriately and add a correct
"// gate-test-proc_macro" comment, proc_macro was added to the whitelist.

We fix this little weakness in the feature gate tidy check and add
the src/test/compile-fail-fulldeps directory to the checked directories.

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