Move hydrostatic test model to a model that's used at runtime

[chapulina]

On #89 there are tests being added for hydrostatics which stripped the model and world of various plugins. This means we now have duplicated configurations in another place.

Ideally, the tests would use the same model that's used at runtime, even if it only tests a subset of it. One idea from https://github.com/osrf/lrauv/pull/89/files#r777306599 would be to add a new intermediate model called tethys_hydrostatic:

tethys ∈ tethys_hydrostatic ∈ tethys_equipped