TST: bare-bones fixture for timedelta array tests
Broken off from #23140.  The fixture needs to be fleshed out, but for now I just want to get it in place, since already other PRs need it.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_datetimelike.py`](https://github.com/jbrockmendel/pandas/blob/8f7d0b50ab076dac9cddaed288403190280f24f4/pandas/tests/arrays/test_datetimelike.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23207?src=pr&el=h1) Report
> Merging [#23207](https://codecov.io/gh/pandas-dev/pandas/pull/23207?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b54e20b4c05727268857dc8cd8acd065f70c6048?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/23207/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23207?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23207   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50950    50950           
=======================================
  Hits        46975    46975           
  Misses       3975     3975
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23207?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/23207?src=pr&el=footer). Last update [b54e20b...87f970c](https://codecov.io/gh/pandas-dev/pandas/pull/23207?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!