Clean up tests, more thorough version testing

[davidism]

• SQLALCHEMY_ENGINE isn't a config key, so the default sqlite:// was being used.
• SQLALCHEMY_DATABASE_URI defaults to sqlite:// anyway, so just remove all these configs
• CustomQueryClassTestCase wasn't registered
• CustomQueryClassTestCase.test_dont_override_model_default wasn't testing the right thing, and was also failing
• CustomModelClassTestCase wasn't registered
• SQLite can have multiple in-memory databases for binds, don't need to create/remove files
• Flask lowest tests against blinker==1.0, which doesn't have connected_to, replace with connect and disconnect
• import flask_sqlalchemy as sqlalchemy is a confusing alias, used fsa instead
• added "lowest" and "current" tox envs for testing dependency versions
• use pytest as the test runner

[davidism]

Ignore the failed ci build, I think it got confused between the initial push and the pr. It does pass: https://travis-ci.org/mitsuhiko/flask-sqlalchemy/builds/121980819.