[Tests] Add tests for mysql and postgres databases

[christopherpickering]

They act differently than sqlite and have had multiple issues reported in the past. It would be wise to add in a test run for the sqlalchemy session on mysql, postgres, oracle (if possible).

see #102 for oracle info.

[darless]

This can be added once #179 is merged in which puts the tests in a tests directory and has a file specifically for SQLAlchemy. We can add tests there for the different SQL backends.

[Lxstr]

Hi @darless thanks for the pr which is merged. I have taken things a fair bit further but still keen to do this kind of broader sql testing. We have this as a little PGSQL example https://github.com/Lxstr/flask-sessions-testing. I'm thinking docker and pre-commit may be helpful. Rather than having to install so many databases.