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.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19509?src=pr&el=h1) Report
> Merging [#19509](https://codecov.io/gh/pandas-dev/pandas/pull/19509?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69cd5fbfd8b57033b0eb280ebc6dd502652f269c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19509/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19509?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19509   +/-   ##
=======================================
  Coverage   91.69%   91.69%           
=======================================
  Files         148      148           
  Lines       48561    48561           
=======================================
  Hits        44530    44530           
  Misses       4031     4031
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19509?src=pr&el=footer). Last update [69cd5fb...1a43bb5](https://codecov.io/gh/pandas-dev/pandas/pull/19509?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 +1

FWIW there are some other `astype` tests that may be in suboptimal locations:

`test_timedelta.test_freq_conversion` (and the Series cases probably belong elsewhere)
`test_ops.test_astype_object`
 thanks!