TST: Extend timedelta64 arithmetic tests to TimedeltaArray
A handful of non-test changes to make the tests pass, all non-test changes are strictly necessary.

Some of the tests are not yet extended, as making the TDA case work will require more invasive changes.
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/core/indexes/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/core/indexes/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/ops.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/core/ops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/conftest.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/tests/arithmetic/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/jbrockmendel/pandas/blob/cb63fe735b97dd6b6a1944396dc47ae0c9c1339a/pandas/util/testing.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@2af56d4`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `94.11%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23642/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23642   +/-   ##
=========================================
  Coverage          ?   92.29%           
=========================================
  Files             ?      161           
  Lines             ?    51489           
  Branches          ?        0           
=========================================
  Hits              ?    47520           
  Misses            ?     3969           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <94.11%> (?)` | |
| #single | `42.32% <67.64%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23642/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.48% <100%> (ø)` | |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23642/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `89.18% <85.71%> (ø)` | |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23642/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `96.44% <96.15%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=footer). Last update [2af56d4...2df7d65](https://codecov.io/gh/pandas-dev/pandas/pull/23642?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase Updated to use `box_with_array` instead of `box_with_timedelta` Following this I think there will be just one more round of DTA/TDA arithmetic test PRs.  Almost there. > Following this I think there will be just one more round of DTA/TDA arithmetic test PRs

The "one more" this was referring to was #23789, but I forgot one other: the follow-up discussed above to implement+test rdiv, rtruediv, etc.  I'm pretty psyched to have this all wrapped up. gentle ping.  follow-up waiting in the wings @jbrockmendel Thanks!