Add proper scenario-based end to end testing

[Half-Shot]

We have some rudimentary unit tests, but what would be good is proper e2e testing of the bot where we can test that the things we want to happen on Matrix actually happen. Having had quite a bit of success with what we did with bridges, my first instinct is to try working on something similar for this (https://github.com/matrix-org/matrix-appservice-irc/tree/develop/spec/e2e).

The harder problem is that the service is currently not especially easy to test, given it depends on a lot of things to exist in the environment rather than passed in. I'm going to have a foray into whether we can split that out enough to run the service in a series of tests, rather than having to do hacks around it.

[Half-Shot]

This was kicked off with #186 but it needs further work.