More comprehensive testing for immersed boundaries

[glwagner]

PRs #1935 and #1936 fix bugs associated with ImmersedBoundaryGrid: the first is a functionality bug that needs a detailed functionality test to catch (for example, testing that budgets are correct with immersed boundaries); the second fixes a more trivial issue with dispatch.

We have a small number of immersed boundary tests; however we need a more comprehensive suite of tests to ensure that immersed boundaries are compatible with various closures and advection schemes.

I think we should use the hydrostatic model for these tests for the time being, because immersed boundaries with non hydrostatic models are still experimental and under development.

cc @whitleyv @jm-c

[glwagner]

I'm closing this issue because I'm judging that it's not of current, timely relevance to Oceananigans development. If you would like to make it a higher priority or if you think the issue was closed in error please feel free to re-open.