Maybe make UVBeam tests more modular

[plaplant]

Some of the tests for the UVBeam object are relatively long, and test multiple aspects. Some of this is for performance reasons, to save duplicated work getting the object to be in the right state for a given test. However, we should still try to be as modular as possible, so that a failed test is more indicative of what exactly went wrong and individual tests are easier to follow. In cases where it doesn't make sense to break tests up, we should add comments explaining the motivation behind keeping things as they are.