Remove orm coupling from tests

I moved the ORM-specific test code from the common files to ORM-specific files. This way we can keep them in one place.
I need to check if tests will pass, as I removed some DM conditions. Everything passed locally, but let's see if Travis confirms.

Also, development for DM is quite painful and I'm happy to drop the support since 1.x, but on the other hand it shows that there are libraries that require some more work than AR and Mongoid, so if we want to let users to develop their own adapters, our code must take these exceptions into account.