Make reference, test argument ordering consistent in test assertions

[mcwitt]

Originally discussed here.

Some test assertions flip the suggested ordering of the reference and test data.

E.g. cases returned by grep -R 'assert_\(allclose\|almost_equal\)(ref[^,]*, [^)]*)':

tests/common.py:                np.testing.assert_allclose(ref_u, test_u, rtol=rtol, atol=atol)
tests/common.py:                np.testing.assert_allclose(ref_du_dl, test_du_dl, rtol=rtol)
tests/common.py:                    np.testing.assert_allclose(ref_du_dp, test_du_dp, rtol=rtol, atol=atol)
tests/test_nblist.py:        np.testing.assert_almost_equal(ref_ctrs, test_ctrs)
tests/test_nblist.py:        np.testing.assert_almost_equal(ref_exts, test_exts)
tests/test_loss.py:    np.testing.assert_allclose(ref, test)
slow_tests/test_mtm.py:        np.testing.assert_almost_equal(ref_prob, test_prob)
slow_tests/test_determinism.py:                    np.testing.assert_almost_equal(ref_U, test_u, decimal=decimals)
slow_tests/test_determinism.py:                    np.testing.assert_almost_equal(ref_U, test_u_selective, decimal=decimals)