REF/TST: Add more pytest idiom to indexing/multiindex/test_xs.py
- [ n/a] xref #24040
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ n/a] whatsnew entry

Hello @simonjayhawkins! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_xs.py`](https://github.com/simonjayhawkins/pandas/blob/3a9415d35f76d40b0ac9e3844ea044e4bcbee8e1/pandas/tests/indexing/multiindex/test_xs.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=h1) Report
> Merging [#24370](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6cf7d9e2606e82fd3934e1dac8119c43f5ee643?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/24370/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24370   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         162      162           
  Lines       51832    51832           
=======================================
  Hits        47839    47839           
  Misses       3993     3993
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.98% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24370?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/24370?src=pr&el=footer). Last update [f6cf7d9...3a9415d](https://codecov.io/gh/pandas-dev/pandas/pull/24370?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/24370?src=pr&el=h1) Report
> Merging [#24370](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6cf7d9e2606e82fd3934e1dac8119c43f5ee643?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/24370/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24370   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         162      162           
  Lines       51832    51832           
=======================================
  Hits        47839    47839           
  Misses       3993     3993
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.98% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24370?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/24370?src=pr&el=footer). Last update [f6cf7d9...3a9415d](https://codecov.io/gh/pandas-dev/pandas/pull/24370?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!