Poisson solver tests use CenterField to represent fields at Faces

[glwagner]

The poisson solver tests have a mysterious property:

Oceananigans.jl/test/test_poisson_solvers.jl

Lines 17 to 20 in 5fbd8cd

	Ru = CenterField(arch, grid, u_bcs)
	Rv = CenterField(arch, grid, v_bcs)
	Rw = CenterField(arch, grid, w_bcs)
	U = (u=Ru, v=Rv, w=Rw)

whereby CenterField is used for Ru, Rv, and Rw --- fields that should be located on faces, and have boundary conditions appropriate for velocity fields.

When these fields are merely relocated to the correct location, the tests fail.

Since by all accounts the Poisson solver is correct, there is probably something wrong with the tests. But we can't really be sure about either while the tests contain an obvious inconsistency. We should probably fix these tests.

@ali-ramadhan, do you remember why we use CenterField for Ru, Rv, and Rw? The code was added in this PR: #834.

[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.