This is similar to #9023 but worse, because it's less of a corner case, and more of a full edge.

Whereas assert_equal "only" treats nans in object arrays incorrectly, its not even possible to put object arrays into assert_allclose at the most basic level:

>>> from numpy.testing import assert_allclose
>>> n = np.array([1], dtype=object)
>>> assert_allclose(n.astype(int), n)
[...]
TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

This makes it unnecessarily hard to write parametric tests for various corner cases (which are already hard to write at the best of times...).