Should tests running Wannier90 be kept?

[greschd]

In order to add a test for #55, I'm adding separated calculation / parser tests (which don't actually run the code) as implemented in aiida-quantumespresso.

The question now is whether the current test, which does run Wannier90, should be kept.

Points in favor of removing it:

• simplifies test setup (Wannier90 install not required)
• tests run much quicker
• can get rid of aiida-pytest dependency

In favor of keeping it:

• easier to check compatibility with new Wannier90 versions

@giovannipizzi @normarivano, opinions?