Tests: Test CMakeTest*Compiler.cmake fallbacks

Add tests that explicitly test the fallback tests of the modules which
test for a functional [Obj]C[xx] compiler, bypassing the ABI tests.
Due to the ABI test short-circuiting added by commit 1d21dd0f7c
(enable_language: Assume compiler works if ABI detection compiles,
2020-05-25, v3.18.0-rc1~93^2), this logic is likely not getting tested
otherwise.