TST: Remove dupe TimdeltaIndex tests from indexes/timedeltas/test_astype.py
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Both classes in `test_astype.py` and [`test_timedelta.py`](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/timedeltas/test_timedelta.py#L19) inherit from `DatetimeLike`, which in turn [inherits from `Base`](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/datetimelike.py#L9), leading to 200+ dupe tests being run.

Modified `test_astype.py` to inherit from `object` instead, removed some unnecessary boilerplate, and removed some additional tests that were specified to override base class tests.
