TST: Reorganize IntervalIndex tests

[jschendel]

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.

[jreback]

thanks @jschendel always love to see reorg on tests :->