Get rid of go-imap-sql weird abstractions

Originally, this test suite was developed for testing go-imap-sql. When I moved it into a separate repository, I left all abstractions used by go-imap-sql it in. This makes it harder to adapt it for other backends.

Notable issues:
* UserDB tests block is not relevant to IMAP functionality and should be removed
* Almost all tests assume that they can create new user accounts freely using the UserDB interface, we should instead provide a callback like initBackend and cleanBackend as current solution is not usable for certain types of backends (proxies, bridges, etc).