TST: split up pandas/tests/test_multilevel.py
- [ ] closes #23782
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n/a ] whatsnew entry

@jreback 
1. i've used fixtures for `frame` and `ymd` instead of copying across the test base class.
2. not moved tests with names matching `getitem` from the `TestSorted` class.
3. frame used in `test_getitem_int` renamed to _frame to avoid clash with fixture.
Hello @simonjayhawkins! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexing/test_multiindex.py`](https://github.com/simonjayhawkins/pandas/blob/7607d5f07a34bfa107c7a0e26fe70c03efc72a9d/pandas/tests/indexing/test_multiindex.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_multilevel.py`](https://github.com/simonjayhawkins/pandas/blob/7607d5f07a34bfa107c7a0e26fe70c03efc72a9d/pandas/tests/test_multilevel.py) !

 ok, can you merge master. i think something is failing as well (the 2.7 should not be a problem any longer). ideally also see if you can move other indexing tests (that you commented about above), even if you have to slightly duplicate whats in the the base class (but do it as fixtures). > i think something is failing as well (the 2.7 should not be a problem any longer)

could have been zip or range...  now added to `from pandas.compat ...` you need to isort imports # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23797?src=pr&el=h1) Report
> Merging [#23797](https://codecov.io/gh/pandas-dev/pandas/pull/23797?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/20ae4543c1d8838f52229830bfae0cc8626801bb?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/23797/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23797?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23797   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files         161      161           
  Lines       51500    51500           
=======================================
  Hits        47528    47528           
  Misses       3972     3972
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |


------

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 yes anything that looks like an indexing tesr thanks @simonjayhawkins  it might make sense to now split pandas/tests/indexing/test_multiindex -> pandas/test/indexing/multiindex/

with a series of files under multiindex logically split. @jreback i recall seeing some discussion on moving towards the functional idiom of pytest but can't seem to find it.

i'll raise a follow on PR and on splitting the test i'll drop the classes as has been done in https://github.com/pandas-dev/pandas/blob/master/pandas/tests/arrays/test_period.py
      