TST: split up pandas/tests/indexing/test_multiindex.py
- [n/a ] follow on from  #23797
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ n/a] whatsnew entry

only whole tests moved, tests have not been split up. this and the removal of classes is left for a follow-on PR.
Hello @simonjayhawkins! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/conftest.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_datetime.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_datetime.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_iloc.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_iloc.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_ix.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_ix.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_loc.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_loc.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_multiindex.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_multiindex.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_panel.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_panel.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_partial.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_partial.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_slice.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_slice.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_sorted.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_sorted.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexing/multiindex/test_xs.py`](https://github.com/simonjayhawkins/pandas/blob/5b50ab137b4a89f0864790ad7beb73a7d1569499/pandas/tests/indexing/multiindex/test_xs.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23912?src=pr&el=h1) Report
> Merging [#23912](https://codecov.io/gh/pandas-dev/pandas/pull/23912?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/43b2dabdcab50e0c83e37b0b8debcd579c539a9f?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/23912/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23912?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23912   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         161      161           
  Lines       51513    51513           
=======================================
  Hits        47554    47554           
  Misses       3959     3959
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <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/23912?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/23912?src=pr&el=footer). Last update [43b2dab...5869d15](https://codecov.io/gh/pandas-dev/pandas/pull/23912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback green! thanks!