5f6796d tests: parametrize "libc++" in tests

Authored and Committed by mprchlik 2 years ago
    tests: parametrize "libc++" in tests
    
    Downstream has no `libc++`, but it can run clang tests with `libstdc++`.
    Parametrize the tests, leaving the actual library to use on their
    respective `main.fmf` files that are easier to overturn in downstream
    repos.
    
        
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