[clang-format] Move comment tests to their own file.

Summary: With a growing suite of comment-related tests, it makes sense to take them out of the main test file. No functional changes.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek, mgorny

Differential Revision: https://reviews.llvm.org/D29713

llvm-svn: 294439