TST: split test_indexing

[jreback]

after #18497 pandas/tests/series/test_indexing.py and its frame equiv are getting large

we could consider splitting these out into a sub-dir, maybe similar to how we split pandas/tests/indexing, which is parially by indexer type, e.g. .iloc/.loc with some split on dtype (datetimes/period).

[almaleksia]

Hi, has it been already taken by someone? If not I would be happy to do it.