[clang-tidy][test] Fix test failure when LLVM_ENABLE_WERROR is set.

After https://reviews.llvm.org/D80531 landed, a subtle bug was introduced where the test would fail if `LLVM_ENABLE_WERROR` was set. This just silences that error so the test case runs correctly, down the line it may be worth not enabling `-Werror` for clang-tidy tests even if the cmake flag is passed.