Should CI be running clang-format on the test directory?

[AaronBallman]

Question: why is the CI running clang-format on test files? (That seems like a really odd decision given that the community doesn't require formatting of their test files and frequently test files can't be formatted the way clang-format wants because of the nature of some tests (like lexing and preprocessing tests).

Originally posted by @AaronBallman in #3158 (comment)