document `assert_allclose` and `pytest.approx`

[IAlibay]

Since these are the favoured assertion methods for the core library, we should document these in testing.rst.

[jandom]

This ticket sounds very similar to https://github.com/MDAnalysis/UserGuide/issues/171 – what's the difference? pytest.approch?

[IAlibay]

@jandom yeah the addition here is pytest.approx. MDAnalysis recommends using approx for single float comparisons and allclose for arrays (it's purely a speed thing, allclose on single numbers is very slow).