DEV: Skip CSS watcher when running QUnit tests and expose more Chrome logs (PR #13390)

There are 2 changes in this PR:

  1. Add a new environment variable called DISCOURSE_SKIP_CSS_WATCHER to disable our stylesheet watcher, and make the qunit:test rake task set this variable on the Unicorn/Rails server it spins up to disable our stylesheet watcher when running the tests because it doesn’t really need it.

  2. Print more Chrome logs (such as network/security errors) to the console.

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