CLN: resolve DeprecationWarning in `pandas/_testing.py` #35942
- [x] closes #35942
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Thanks for the PR. Can you confirm that this is the correct fix? On master I see this

```
> /Users/taugspurger/sandbox/pandas/pandas/_testing.py(790)assert_index_equal()
-> diff = np.sum((left.values != right.values).astype(int)) * 100.0 / len(left)
(Pdb) left
Index([<NA>, NaT], dtype='object')
(Pdb) left.values
array([<NA>, numpy.datetime64('NaT')], dtype=object)
(Pdb) right.values
array([<NA>, NaT], dtype=object)
(Pdb) pp np.no
np.nonzero    np.not_equal
(Pdb) pp np.not_equal(left.values, right.values)
*** TypeError: boolean value of NA is ambiguous
```

Are we raising a TypeError but not seeing it since the test is xfailed? > Thanks for the PR. Can you confirm that this is the correct fix? On master I see this
> 
> ```
> > /Users/taugspurger/sandbox/pandas/pandas/_testing.py(790)assert_index_equal()
> -> diff = np.sum((left.values != right.values).astype(int)) * 100.0 / len(left)
> (Pdb) left
> Index([<NA>, NaT], dtype='object')
> (Pdb) left.values
> array([<NA>, numpy.datetime64('NaT')], dtype=object)
> (Pdb) right.values
> array([<NA>, NaT], dtype=object)
> (Pdb) pp np.no
> np.nonzero    np.not_equal
> (Pdb) pp np.not_equal(left.values, right.values)
> *** TypeError: boolean value of NA is ambiguous
> ```
> 
> Are we raising a TypeError but not seeing it since the test is xfailed?

There are 4 xfail at `pandas/tests/indexes/test_index_new.py:99`. Before the change, it was an
`AttributeError: 'bool' object has no attribute 'astype'`. After the change, it became `TypeError: boolean value of NA is ambiguous`. Right, do we want that TypeError? What's the desired behavior? > Right, do we want that TypeError? What's the desired behavior?

Not sure about this. Might need some discussion. xref #31884 This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this.