TST: Allow assert_numpy_array_equal to compare nan
Related to #10373.  `pandas` `assert_array_equal` needs to be updated to perform `nan` comparison properly.

```
import numpy as np
import pandas as pd
import pandas.util.testing as pdt
import numpy.testing.utils as npt

npt.assert_array_equal(np.array([1, np.nan, 3]), np.array([1, np.nan, 3]))
# no AssertionError

pdt.assert_numpy_array_equal(np.array([1, np.nan, 3]), np.array([1, np.nan, 3]))
# AssertionError: [  1.  nan   3.] is not equal to [  1.  nan   3.].
```
