TST: Condense tests in test_interval_new.py and add new test based on TODO
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Combined a some pairs of similar tests into single tests, e.g. `test_get_indexer_with_interval_multiple_queries` and `test_get_indexer_with_interval_single_queries` are essentially testing the same functionality but with different input to `get_indexer`, so just needed a couple of superficial changes to combine into a single test.

I also added one test based on a TODO, `test_get_indexer_errors`, which verifies that `get_indexer` raises if the `IntervalIndex` is overlapping.  Note that this is consistent with `get_indexer`  raising for other indexes if the index is non-unique; for intervals uniqueness is not enough to ensure unique query results, as a scalar query against an overlapping `IntervalIndex` can return non-unique indices.

I've got all these tests to pass on my WIP branch implementing the new behavior, with the exception of the `slice_locs` tests, so will hopefully have a PR for that soon.
