Make tests for non-stokes I sources

[bhazelton]

[jpober]

We might already have this. pyradiosky has a more detailed issue for analytically calculated coherencies, but if this is simply a question of testing our visibility calculation against a known coherency (with non-zero off diagonal terms), then this could already have been met.

[jpober]

pyuvsim needs a better test. It doesn't really check the answers, just that the functionality is useable.