Generating charges in the tests

I noticed that we generate partial charges in some protocol tests, such as test_dry_run_default_vacuum. In this case, we repeatedly generate the same charges for each sampler type, which is an expensive operation when the charge generation itself is not what is being tested. I think it would be best to have a single explicit test which checks that a protocol is correctly generating charges, and then for all other dry runs to use pre-calculated charges to keep the tests quick, maybe this is something we should come up with a best practice for and add it to the dev guide we plan on making?

Start with openfe/tests/protocols/openmm_rfe/test_hybrid_top_protocol.py and test_dry_run_default_vacuum, then inspect the other protocol tests that generate partial charges. Identify one explicit charge-generation test and update unrelated dry runs to use pre-calculated charges; done means the charge behavior remains covered while the tests avoid repeated charge generation.