REF/TST: Add more pytest idiom to indexing/multiindex/test_getitem.py
xref #24040

for this pass:

- the Test classes have been removed.

- `catch_warnings`  used for the `.ix` deprecation replaced with `@pytest.mark.filterwarnings("ignore:\\n.ix:DeprecationWarning")` at test level.

cc @jreback @gfyoung 