TST: Fix improperly defined tests in test_interval_new.py
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I've been working over the new `IntervalIndex` behavior specs and found some issues with the tests for the new behavior.  Wanted to make the requisite changes separately so the new behavior PR is as straightforward as possible (will open a WIP PR for it at some point soonish).

Specifically fixed a few instances of the following:
- Combined instances of two parametrize's that should instead be one pairwise parametrize (i.e. test was doing the product of all parameters instead of pairwise parameters)
- Used the `closed` fixture instead of an equivalent explicit parametrize
- Fixed a few instances of misplaced parentheses
- `expected_result` --> `expected`
- parametrized one test where obvious
