[OpenMP][omptest] Improve CMake and address review comments (#159416)

Avoid explicit ABI breaking check deactivation
Replace whole-archive linking with dedicated build of GoogleTest lib

Addresses remaining post-merge review comments of
https://github.com/llvm/llvm-project/pull/154786

GitOrigin-RevId: d5fea7e4f0f2be1f0eae7a3f9e59860632134383