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.
It's not necessary to store the tests in a separate repository, since
other packages can just reference the tests in this repo.
Also update, the other tests to pull from git repos in the rpms namespace.
