Exclude lint test files from clang-format

[ChristianKuehnel]

from: https://reviews.llvm.org/D72715

Maybe also check if there are other file types that should be excluded...

[kadircet]

Yeah actually I had also reported this to @metaflow last week (should've filed it here instead I suppose).

There is actually a .clang-format file inside clang/test/ directory, but for some reason the clang-format being run during the premerge builds doesn't pick that up, hence the false positives.

[njames93]

This seems to be the same issue as #92

[ChristianKuehnel]

@njames93 yes, thx. Closing this ticket as duplicate.