TST: Fix test for datetime categorical
Related to #10465, but different part.

Current `test_groupby_datetime_categorical` in `test_groupby.py` is incorrect, the actual result returns `CategoricalIndex` as level 0, otherwise expected result uses `DatetimeIndex` as level 0. Changed to use the same dtype and added explicit comparison.
### Actual Result (current test case)

```
levels = pd.date_range('2014-01-01', periods=4)
codes = np.random.randint(0, 4, size=100)
cats = pd.Categorical.from_codes(codes, levels, name='myfactor', ordered=True)
data = pd.DataFrame(np.random.randn(100, 4))
grouped = data.groupby(cats)
desc_result = grouped.describe()
desc_result.index.get_level_values(0)
# CategoricalIndex([2014-01-01T09:00:00.000000000+0900,
#                  ...
#                   2014-01-04T09:00:00.000000000+0900],
#                  categories=[2014-01-01 00:00:00, 2014-01-02 00:00:00, 2014-01-03 00:00:00, 2014-01-04 00:00:00],
# ordered=True, name=u'myfactor', dtype='category')
```
### Expected Result (current test case)

It must be `CategoricalIndex`.

```
idx = cats.codes.argsort()
ord_labels = np.asarray(cats).take(idx)
ord_data = data.take(idx)
expected = ord_data.groupby(ord_labels, sort=False).describe()
expected.index.names = ['myfactor', None]
expected.index.get_level_values(0)
# DatetimeIndex(['2014-01-01', '2014-01-01', '2014-01-01', '2014-01-01',
#               ...
#               '2014-01-04', '2014-01-04', '2014-01-04', '2014-01-04'],
#               dtype='datetime64[ns]', name=u'myfactor', freq=None, tz=None)
```

lgtm, merge when ready
 Thanks, merging.
