Add testing guidelines

[jsjoeio]

I would like to add a section to MAINTAINING which adds Testing Guidelines. This should explain:

• how testing works in code-server
• where to put your tests
• how to figure out what needs more tests
• how tests work in CI

[bpmct]

We might want to add sections/instructions for manually testing things that don't have tests yet (such as the postinstall script or things to test post-release on different operating systems)

[jsjoeio]

Good idea!