TST: Reorganize IntervalIndex tests
There shouldn't be any behavioral changes here: mostly just moved things around and created new files for better organization.  I tried to do separate granular commits, so looking at each commit individually might make things a little more obvious if there are concerns.

Summary:
- Removed `test_interval_new.py`
- Created `test_indexing.py` for tests specific to indexing methods
   - Nearly all of `test_interval_new.py` is here
   - Also moved relevant tests from `test_interval.py` here
- Created `test_base.py` for tests inherited from the common `Base` class
  - Only tests related to common tests should be placed here (i.e. overriding, skipping)
- Created `test_format.py` for formatting related methods

All of the above is generally consistent with how the datetime/period indexing tests are organized.