TST: Split tests/indexes/interval/test_interval.py into separate files
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Since we now have an interval subdirectory, seems logical to split `test_interval.py` into separate files, similar to what has been done for `DatetimeIndex`, `PeriodIndex`, and `TimedeltaIndex`. 

Just made very basic changes here, moving things at the class level, and didn't break apart any classes.  All of the tests should functionally be the same.

Summary:

- Moved `TestIntervalRange` class from `test_interval.py` to `test_interval_range.py`
- Moved `TestIntervalTree` class from `test_interval.py` to `test_interval_tree.py`
  - Changed a few `for` loops to `@pytest.mark.parametrize`
  - Converted a `lambda` to an actual function (my linter was complaining about PEP8 E731)
- No changes to `test_interval_new.py` 
   - Should still only cover tests in `test_interval.py`

Down the road we could probably split `test_interval.py` into smaller components, much like what was done for `DatetimeIndex`, `PeriodIndex`, and `TimedeltaIndex`.  Might want to wait until the `_new.py` files are fully addressed though.  Didn't look like there were any high level changes like this that could be made in `tests/indexing/interval/`.

