Enable two-center case in test_poisson_bvp_gives_the_correct_laplacian

[Phantom8420]

This case was commented out with # TODO: Couldn't get the following test to pass. I ran it against current master (two Gaussians at (0,0,0) and (1,1,1), default params) and it passes: max abs error ~6e-3 on grid points and ~3e-14 on random points, both under the existing atol=1e-2. Looks like it was fixed as a side effect of other work since this was written, just never re-checked. Uncommenting it.

[PaulWAyers]

It looks like a good contribution to me. I'd want @marco-2023 and/or @Ao-chuba to confirm that this test now no longer needs to be commented out, however, as I'm not especially familiar with this part of the code.

[Phantom8420]

Gentle bump. CI is green on all platforms here, including the case that was commented out. Happy to add anything that would help confirm it's safe to enable.

[Ao-chuba]

@Specter842
Based on my benchmarking work #315, the BVP solver has been improved since then. we confirmed this when testing the original water dimer from #215 and the BVP solver now handles it without crashing, so enabling this two-center Laplacian test is consistent with all of this. If CI is green, I think it is safe to merge.
also there is no logic change in the code so its a minimal change but correct