REF/TST: Add more pytest idiom to resample/test_base.py
- [ n/a] xref #17806
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ n/a] whatsnew entry

in this pass:
- restructure fixtures in order to parametrize base class tests
- remove test base class

Hello @simonjayhawkins! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/resample/conftest.py`](https://github.com/simonjayhawkins/pandas/blob/a6e205d3d0ae2ce964a55bea0be1d5fba104a32b/pandas/tests/resample/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_base.py`](https://github.com/simonjayhawkins/pandas/blob/a6e205d3d0ae2ce964a55bea0be1d5fba104a32b/pandas/tests/resample/test_base.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_period_index.py`](https://github.com/simonjayhawkins/pandas/blob/a6e205d3d0ae2ce964a55bea0be1d5fba104a32b/pandas/tests/resample/test_period_index.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=h1) Report
> Merging [#24324](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3675c29a7bf8714d3f83d1c6a854325702f81969?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/24324/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24324   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files         162      162           
  Lines       51831    51831           
=======================================
  Hits        47833    47833           
  Misses       3998     3998
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24324?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/24324?src=pr&el=footer). Last update [3675c29...a6e205d](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=h1) Report
> Merging [#24324](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3675c29a7bf8714d3f83d1c6a854325702f81969?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24324/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24324      +/-   ##
==========================================
- Coverage   92.28%   92.28%   -0.01%     
==========================================
  Files         162      162              
  Lines       51831    51831              
==========================================
- Hits        47833    47832       -1     
- Misses       3998     3999       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24324/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.48% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24324?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/24324?src=pr&el=footer). Last update [3675c29...4857ece](https://codecov.io/gh/pandas-dev/pandas/pull/24324?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 let me have a look locally i'm not happy with the restructuring of the fixtures. trying a different approach.