Create additional test grouping

**Is your feature request related to a problem? Please describe.**
We currently try to run GPU tests in all cases (not great long-term plan), and are now running into some dependency issues like here: https://github.com/OpenFreeEnergy/openfe/issues.

**Describe the solution you'd like**
We should make more use of `pytest.mark` and the like to better control what tests get run.
