Fixturize tests/frame/test_join.py
One more steps towards #22471.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=h1) Report
> Merging [#25639](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/26cfa288edfc2db85d23d3e23cb6ce3888675963?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/25639/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25639   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         173      173           
  Lines       52968    52968           
=======================================
  Hits        48339    48339           
  Misses       4629     4629
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25639?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/25639?src=pr&el=footer). Last update [26cfa28...5ca3e2f](https://codecov.io/gh/pandas-dev/pandas/pull/25639?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/25639?src=pr&el=h1) Report
> Merging [#25639](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/26cfa288edfc2db85d23d3e23cb6ce3888675963?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/25639/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25639   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         173      173           
  Lines       52968    52968           
=======================================
  Hits        48339    48339           
  Misses       4629     4629
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25639?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/25639?src=pr&el=footer). Last update [26cfa28...5ca3e2f](https://codecov.io/gh/pandas-dev/pandas/pull/25639?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks