TST: Move fully passing tests from test_interval_new to test_interval
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Some of the `test_interval_new.py` tests are fully passing, so moving them to `test_interval.py` to ensure that new changes don't break them.

Added a couple of TODO's to the new tests related to checking non-monotonic/overlapping/dupe indexes, which generally weren't covered in the new or old tests (with one noted exception).  For the most part existing new/old tests just check the non-overlapping monotonic case.

Made some additional minor modifications to `test_interval_new.py`: cleaned up a couple tests by using `parametrize`, and fixed a few instances of a misplaced parenthesis.