CI: bail e2e tests if a test instance is already running
Created by: chrismwendt
See original report of CI failures and investigation: https://sourcegraph.slack.com/archives/C07KZF47K/p1559640089012800
Somehow an e2e test from 2 weeks ago kept running and subsequent e2e test runs would inadvertently reuse the same instance, which presumably accumulated state and broke some Percy tests.