CLN: tests.indexes.test_setops.py
I'm experimenting with subclassing NumericIndex and using the test suite to check if everything passes ok (by adding the new subclass to `indices_dict` in `conftest.py` and making a `makeXIndex` func in `_testing.py.

`COMPATIBLE_INCONSISTENT_PAIRS ` in `test_setops.py` is very tied to specific index classes, so is difficult to use on new index classes. So here's a PR to make this part more friendly to testing subclasses by making it test dtypes instead of index class.