Add nightly browser extension test runs

Created by: lguychard

We should improve our browser extension tests so that we can:

• Catch regressions caused by GitHub DOM changes (this may be as simple as running our regular browser extension tests more often than just on bext release)
• Catch visual regressions such as https://sourcegraph.slack.com/archives/CHXHX7XAS/p1579741455063500

This should take the form of:

• Nightly browser extension tests against github.com
• Nightly browser extension tests against gitlab.com
• Nightly native integration tests agains gitlab.com
• Upload coverage under an extra flag

Tests should cover:

• Code intelligence on all supported code views (file, commit, pull request in unified/split view...)