Add tests for Face-Face halo filling

[navidcy]

This PR adds tests for Face-Face halo filling and ensures that the fill_halo_regions! for Face-Face fields are done correctly.

After this PR a fill_halo_regions! works on a Face-Face field on a ConformalCubedSphereGrid with the exception of the 2 corner points that belong in the halo regions and do not correspond to any interior point!

If those corner points are filled manually

Oceananigans.jl/validation/multi_region/cubed_sphere_tracer_advection.jl

Lines 32 to 53 in b0ff6c4

	# Note: fill_halo_regions! works for (Face, Face, Center) field, *except* for the
	# two corner points that do not correspond to an interior point!
	# We need to manually fill the Face-Face halo points of the two corners
	# that do not have a corresponding interior point.
	for region in [1, 3, 5]
	i = 1
	j = Ny+1
	for k in 1:Nz
	λ = λnode(i, j, k, grid[region], Face(), Face(), Center())
	φ = φnode(i, j, k, grid[region], Face(), Face(), Center())
	ψ[region][i, j, k] = ψᵣ(λ, φ, 0)
	end
	end
	for region in [2, 4, 6]
	i = Nx+1
	j = 1
	for k in 1:Nz
	λ = λnode(i, j, k, grid[region], Face(), Face(), Center())
	φ = φnode(i, j, k, grid[region], Face(), Face(), Center())
	ψ[region][i, j, k] = ψᵣ(λ, φ, 0)
	end
	end

Then everything seems fine.

(This is of course not a final solution but I wanted to make a note here.)

(work done with @siddharthabishnu)