[llvm] [Flang-RT][Offload] Always use LLVM-built GTest (PR #143682)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Thanks @Meinersbur ! We (Linaro) will report back if we observe it still happening, but I hope not.

https://github.com/llvm/llvm-project/pull/143682