TST: timeseries tests reorg
Datetime
------------
- [x] We have *some* DatetimeIndex tests in [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries.py) and some in [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/test_datetimelike.py) and *a lot* [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_base.py) (PR #15266) 

we can separate these out to a single file in ``pandas/tests/indexes/test_datetime.py``
The ``Base`` class (which powers lots of tests for the sub-classes ``DatetimeIndex``, ``PeriodIndex``, ``TimedeltaIndex``), can move to ``pandas/tests/indexes/tests_datetimelike.py`` (and merge with the existing ``Datetimelike`` class).

- [ ] create a ``pandas/tests/scalar/test_timestamp.py`` with ``Timestamp`` specific tests
There are a lot of tests also in [tseries/test_tslib.py](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_tslib.py), that can be merged into ``test_timestamp.py``

- [ ] the residuals (from ``tests_timeseries.py`` are Series tests speficially, these can move to ``pandas/tests/series/test_timeseries.py``

``Legacy Tests``
---------------
- [ ]  [timeseries_legacy](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_timeseries_legacy.py) need a look, but can prob be deleted

``Period``
----------
- [ ] same idea with Period, the actual ``Period`` tests can be moved to ``tests/scalar/test_period.py``, but move the index tests to ``tests/indexes/test_period.py`` (which can be stripped from [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/test_datetimelike.py) and [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_base.py))

``Timedelta``
-----------
- [ ] same idea with Timedelta, the actual ``Timedelta`` tests can be moved to ``tests/scalar/test_timedelta.py`` but move the index tests to ``tests/indexes/test_timedelta.py`` (which can be stripped from [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/test_datetimelike.py) and [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/tests/test_base.py))
