Add testing documentation

[kylekz]

As per some issues I've finally sorted out (tenancy/multi-tenant#906), wrote up some documentation on the testing process. I'm not much of a writer, nor do I even know a lot about the testing process, I just finally got this working on my own project.

I see #74 is here, however it seems a little out of date. Figured I'd just make a mind-dump of what I've got.

[mikebronner]

@xKairu Thank you for detailing the process of setting up tests and providing a sqlite driver! I wanted to ask you if the way you documented this is still working, as I am not able to get tests to run with 3rd-party packages declared in the config force-tenant-connection-of-models. It seems to want to run those models under the system connection no matter what.

Update: I was able to work through this with the help of the following: https://stackoverflow.com/questions/63206165/setting-up-testing-in-laravel-project-using-hyn-multi-tenant