Use FileCheck in new clang-tidy/infrastructure/config-files.cpp tests

The test was failing spuriously on some machines [1]. Converting from
grep to FileCheck makes it easier to see what's happening, and also grep
is discouraged in tests [2].

Also drop the negative test: checking for the expected output is better.

 1. https://crbug.com/338535754
 2. https://llvm.org/docs/TestingGuide.html#:~:text=.%20%5BThe-,usage%20of%20grep,-in%20RUN%20lines