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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I presume you'll get someone offload related to review later.

The errors we saw on the bots I think are possible with offload too, but I've no hard evidence for that. Keeping them all consistent is good anyway.

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