get rid of the None engine tests and just have a separate test that ensures how that gets bound to a particular engine #26662 (comment)
- [x] xref https://github.com/pandas-dev/pandas/pull/26662#discussion_r291258182, https://github.com/pandas-dev/pandas/pull/26543#issuecomment-497126487, #26784
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] I've replaced `None` engine tests with corresponding separate test for default engine value. I also excluded `test_bad_engine_raises` from `TestReaders` class.
