My goal is to get `util.testing.assert_frame_equal` to compare as equal DataFrames which would be equal except for NaNs in their index. My approach is a little different than the one discussed in https://github.com/pydata/pandas/issues/5183. 

I think perhaps the root of the problem is that there is no tool to easily check when two NumPy arrays are equal except for NaNs. So I've tried to create one which could be a drop-in replacement for `np.array_equal` except that it treats NaNs in corresponding locations as equivalent.