[clang-tidy] NFC Consolidate test absl::Time implementation

Summary: Several tests re-implement these same prototypes (differently), so we can put them in a common location.

Patch by hwright.

Reviewers: JonasToth

Reviewed By: JonasToth

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D55540

llvm-svn: 348840