Add a more strict test for OpenMP+nOS-V

In this test we ensure that the worksharing region is running inside a
nOS-V task, so we know that we are not using the vanilla OpenMP by
accident.

We also keep the previous test test/compilers/clang-openmp.nix as-is, so
we can check that the compiler injects the nosv library dependency in
the final binary on its own.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>