#14854: DatetimeIndex compiled together in test_datetime.py
### Partially completes #14854 

Grouped all the ```DatetimeIndex``` related tests under one file - ```pandas/tests/indexes/test_datetime.py```.

### Tests reorganized

There were some tests in other classes which could be shifted to the as well. From **```pandas/pandas/tseries/tests/test_timeseries.py```**

- [test_pass_datetimeindex_to_index](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py#L326) from class ```TestTimeSeries```
- [test_datetimeindex_accessors](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py#L3972) from class ```TestDatetime64```
- [test_datetimeindex_diff](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py#L4111) from class ```TestDatetime64```
- [test_nanosecond_field](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py#L4105) from class ```TestDatetime64```
- [test_datetimeindex_constructor](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py#L4146) from class ```TestDatetime64```

1. Should these also be included in the ```test_datetime.py``` in their respective classes or let them be?

2. In **```pandas/tests/indexes/test_datetimelike.py```** there were some tests which were using the ```DatetimeLike``` class defined there. I could import that class in the ```test_datetime.py``` and transfer the whole ```TestDatetimeIndex``` but should it be done?


### Mostly passes ``git diff upstream/master | flake8 --diff``

```
./pandas/tests/indexes/test_datetime.py:1121:5: F811 redefinition of unused 'test_intersection' from line 572
./pandas/tests/indexes/test_datetime.py:1751:5: F811 redefinition of unused 'test_union' from line 589
```
3. Should I merge the two ```test_intersection``` and ```test_union``` functions?


## Further TODOs from the following discussion and the #14854 under datetime

- [x] Split ```datetimes/test_datetime.py``` into smaller chunks

- [x] In ```datetimes/test_misc.py``` lines from 167-202 should go under timestamp related tests (?)

- [x] Transfer of function (```test_datetimeindex_accessors```) from class ```TestDatetime64``` in ```datetimes/test_misc.py``` to class ```TestDatetimeIndexOps``` of ```datetime/test_ops.py``` (?)

- [x] Function ```test_round``` from ```datetimes/test_datetime.py``` to ```pandas/tests/scalars/test_timestamp.py```. (and test_timedelta.py). So anything specifically Timestamp related will go there.
