rust private

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. Part of issue #39059 .