[clang-tidy][test] Allow specifying potentially unused suffixes

If a check-suffix is only required for a CHECK-FIXES or CHECK-MESSAGES. check_clang_tidy will pass the prefixes CHECK-FIXES<...> and CHECK-MESSAGES<...> to FileCheck.
This will result in a FileCheck failing because of an unused prefix.

This addresses the problem by not passing unused prefixes. Its also possible to fix this be passing `--allow-unused-prefixes` flag to FileCheck, but seeing as we have already done the legwork in the script to see its unused, this fix seems the better way to go.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D97322