TST: Finish groupby aggregate cleanup
Continuation of #18931 

* Converted class-based setup fixtures to pytest fixtures
* Consolidated all imports at the top of the file

`test_aggregate_item_by_item` has an unused DataFrame in the test so cleaned that up as well

Please let me know if you need me to make any changes.
lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19094?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@856c92b`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19094/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19094?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19094   +/-   ##
=========================================
  Coverage          ?   91.53%           
=========================================
  Files             ?      148           
  Lines             ?    48688           
  Branches          ?        0           
=========================================
  Hits              ?    44567           
  Misses            ?     4121           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <ø> (?)` | |
| #single | `41.63% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19094?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/19094?src=pr&el=footer). Last update [856c92b...f46b2bd](https://codecov.io/gh/pandas-dev/pandas/pull/19094?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback we are green! thanks @alysivji nice PR!