REF: tests.indexes.numeric.py
This PR ensures that all index instantiation calls in `tests.indexes.numeric.py` use the `_holder` class variable instead of the actual index class. This will make it easier to extend the existing tests to tests for the new `NumIndex` in #41153.

Also renames `_holder` to `_index_cls` for all of `tests.indexes`. I think this is a more descriptive name.
