[llvm] [unittest] Allow LLVM unit test to run under a wrapper program. (PR #66821)

[via llvm-commits]

zeroomega wrote:

I am a bit hesitated if I should use `LLVM_GTEST_RUN_UNDER` to eventually allow all gtest based unit test can be redirected to a wrapper program or treat each component individually to just use "LLVM_GTEST_UNITTEST_RUN_UNDER" for this patch. We can add "LLVM_GTEST_CLANG_TEST_RUN_UNDER" later for clang and "LLVM_GTEST_CLANG_TOOLS_TEST_RUN_UNDER" for clang-tools. 

In the use case we have, we want to run googletest based unittest on a cross compiled target platform. In this case, all gtest based unittest should be invoked under the wrapper program, so a global `LLVM_GTEST_RUN_UNDER` options make more sense.


https://github.com/llvm/llvm-project/pull/66821