Add first high-level integration test for Executor

Created by: mrnugget

I present to you: the first high-level integration test to make sure that executing steps works.

Am I proud of this? Yes and no. Yes, because it gives me confidence that something works and it is a nice integration test. No, because, well, look, I created a little Docker "emulator" in Bash and I'm not sure how I feel about that now.

Edit: also, let me add: it creates a ZIP file in memory, mocks the Sourcegraph backend, actually runs commands in a temp directory and allows us to assert that changes were made to the files!