TST: split up pandas/tests/test_resample.py
- [n/a ] precursor to #17806
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n/a ] whatsnew entry

Hello @simonjayhawkins! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_resample.py`](https://github.com/simonjayhawkins/pandas/blob/da3f61761caefb080d80de7ef465f4909e1190e4/pandas/tests/resample/test_resample.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_resampler_grouper.py`](https://github.com/simonjayhawkins/pandas/blob/da3f61761caefb080d80de7ef465f4909e1190e4/pandas/tests/resample/test_resampler_grouper.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_time_grouper.py`](https://github.com/simonjayhawkins/pandas/blob/da3f61761caefb080d80de7ef465f4909e1190e4/pandas/tests/resample/test_time_grouper.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23872?src=pr&el=h1) Report
> Merging [#23872](https://codecov.io/gh/pandas-dev/pandas/pull/23872?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1f02bf240c3d0d3da338af868d056bfc169b28c2?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/23872/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23872?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23872   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         161      161           
  Lines       51500    51500           
=======================================
  Hits        47533    47533           
  Misses       3967     3967
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <ø> (ø)` | :arrow_up: |
| #single | `42.42% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23872?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/23872?src=pr&el=footer). Last update [1f02bf2...fe2153e](https://codecov.io/gh/pandas-dev/pandas/pull/23872?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 i wouldn’t actually change the class structure / fixtures now
leave that for another pass
 @jreback 

> i wouldn’t actually change the class structure / fixtures now
leave that for another pass

OK, i'll just rename _simple_ts etc and leave it at that?

i've changed the checklist from closes to precursor to  yeah a change like this is hard enough - first pass is just reorg minimally and rename

then can come back later and clean  @jreback The three files with the classes changed so far were not inheriting from the base class so the changes were relatively small. should i leave these changes in or revert those three commits? thanks @simonjayhawkins nice split! sure happy to have a followup to address questions / simplify / use fixtures here