TST/CLN: clean up indexes/multi/test_unique_and_duplicates
Splitting up #21645

* Added tests for `duplicated`, including a fixture for a `MultiIndex` with duplicates
* Broke up a huge test (`test_duplicates`) into smaller chunks
* removed a test (`test_unique_na`) that was for an `Index` (not `MultiIndex`), and exists verbatim in `tests/indexes/common.py`
* moved `test_duplicate_level_names_access_raises` to its appropriate module
