[llvm] [flang-rt] Disable tests by default when cross-compiling (PR #208926)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

Could you add a comment that it is because `NonGTestTesting` uses `LINK_TO_LLVM` https://github.com/llvm/llvm-project/blob/308393ede14cc8533d1be122b5df88bf9ba4173b/flang-rt/lib/Testing/CMakeLists.txt#L14

(eventually I think all the unittests should use GTest).

https://github.com/llvm/llvm-project/pull/208926