[Runtimes] If LLVM_INCLUDE_TESTS=On depend on gtest

Summary: If we are building the tests for the runtimes we should make them depend on gtest so that gtest is built and ready before we run any of the check-* targets.

Reviewers: phosek, compnerd

Reviewed By: compnerd

Subscribers: mgorny, winksaville, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62269

llvm-svn: 361436