Add integration tests for important commands

[pjcdawkins]

Tested on the GitLab side

Rationale:

• Go seems to be fine at running these kinds of tests, and it removes the need to worry about Python and Selenium, which have not been working (on GitLab), and which represent unnecessary barriers to testing on other platforms.
• Go allows easy mocking of API servers to test particular commands.

Awkwardnesses were:

• I set cmd.Dir = os.TempDir() when running a command: so that the legacy CLI doesn't do unpredictable things with the "current project".
• I added the chi router package as a dependency for the test mocks. That probably does not affect the production build (on this branch, it's about 1KB smaller for me).

Follow up steps could be:

• Build or download PHP and the Legacy CLI on GitHub Actions, so that the integration tests can be run there.
• Test on multiple operating systems, such as Linux, Windows and MacOS.

[akalipetis]

Really nice one, this will help us on so many fronts 🎉

I have just a couple of questions, the rest of the code is generally looking great, I put most of my focus on the "testing infra" code.

[akalipetis]

LGTM