clang-tidy/add_new_check.py: Adapt to use %check_clang_tidy in tests

Summary: Adapt clang-tidy/add_new_check.py according to commit r251010 "Add %check_clang_tidy and %clang_tidy_diff"

Reviewers: alexfh, sbenza, bkramer, aaron.ballman

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D14049

llvm-svn: 251355