TST: add tests/arrays/interval/test_constructors.py
`IntervalArray` construction wasn't tested very well previously. This adds a `test_constructors.py` file to the `tests/arrays/interval` directory. The test file is the test file from pandas/tests/indexes/interval/test_constructors.py (after #49786) adapted to test `IntervalArray`s instead of `IntervalIndex`es.

Also preparation for fixing an issue in #49560.
can any of this be de-duplicated with the analogous index tests? This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this.