REF/TST: Add more pytest idiom to tests/resample
xref #17806
follow-on from #24013

This pass replaces imports from tests/resample/test_base.py with fixtures and only parametrizes tests where necessary to accomplish this.

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/148470870e0aaae8e558b90a49930bef167625e5/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/148470870e0aaae8e558b90a49930bef167625e5/pandas/tests/resample/test_base.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_datetime_index.py`](https://github.com/simonjayhawkins/pandas/blob/148470870e0aaae8e558b90a49930bef167625e5/pandas/tests/resample/test_datetime_index.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_period_index.py`](https://github.com/simonjayhawkins/pandas/blob/148470870e0aaae8e558b90a49930bef167625e5/pandas/tests/resample/test_period_index.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=h1) Report
> Merging [#24230](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c994e98c0e216622ad025c0c9e28421a4a3ee6af?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/24230/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24230   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51763    51763           
=======================================
  Hits        47733    47733           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <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/24230?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/24230?src=pr&el=footer). Last update [c994e98...1484708](https://codecov.io/gh/pandas-dev/pandas/pull/24230?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/24230?src=pr&el=h1) Report
> Merging [#24230](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2f6d682a06573e0aab64ae90eeaba216f224db12?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/24230/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24230   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51785    51785           
=======================================
  Hits        47759    47759           
  Misses       4026     4026
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24230?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/24230?src=pr&el=footer). Last update [2f6d682...693f84a](https://codecov.io/gh/pandas-dev/pandas/pull/24230?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. can you merge master and ping on green as just merged a resample bug fix. @jreback green! thanks @simonjayhawkins 