[clang-tidy] Add check for underscores in googletest names.

Summary: Adds a clang-tidy warning for underscores in googletest names.

Patch by Kar Epker!

Reviewers: hokein, alexfh, aaron.ballman

Reviewed By: hokein

Subscribers: Eugene.Zelenko, JonasToth, MyDeveloperDay, lebedev.ri, xazax.hun, mgorny, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D56424

llvm-svn: 352183