Fix errors causing tests to fail when run from the UI

[maxbrunsfeld]

This fixes some errors and noise in the console that has been occurring for me when running the tests in the UI.

1. There was a test in atom-environment-spec.js that emitted an auto-update-related event in the main process. This caused exceptions to be thrown in all of your other Atom windows. I found that this code path is now covered by other tests, so I simply removed the offending test.
2. Our strategy for subscribing to multiple different ipc messages in application-delegate.js entailed adding many individual ipc listeners, resulting in frequent EventEmitter leak detected warnings. I've refactored this code to use a single event listener.

[daviwil]

Quick question about quick-resetting project paths: is there any project state that might not be cleared out if calling reset() with the existing paths results in basically a no-op?

[maxbrunsfeld]

I decided to remove my changes regarding the project paths. This was an attempt to fix a problem that had a deeper cause: a performance regression in libgit2 made instantiating GitRepository objects much more expensive. I have now downgraded git-utils to avoid that performance regression, so it's no longer necessary to avoid this code path so carefully.

[maxbrunsfeld]

The Circle CI failure is due to us no longer using Circle CI, and the VSTS windows failure is an unrelated flaky test.