General tests cleanup and increase coverage

[ijpulidos]

While working on #1007 it was noted that there is code in the tests that never gets executed but they might be a good starting point for new and useful tests. We need to work on a cleanup of the tests and identify the parts of the code base that are not covered by tests.

[ijpulidos]

For the PointMutationExecutor check https://github.com/choderalab/perses/pull/1030#discussion_r900267666 . This can be accomplish by using pytest fixtures and avoiding redundant code and efforts. In general, we want that in many of our tests.

[ijpulidos]

We can parametrize the different combinations in test_name and use_point_energies for run_unsampled_endstates_energies in test_unsampled_endstate_energies_GPU test.