Run bext e2e tests on all branches, against a local instance

Created by: lguychard

Adds e2e tests for setting the Sourcegraph URL through the options page, which in turns makes it possible to run the browser extension e2e tests against a local instance in CI. This would allow to run the e2e tests on all branches.

Still have to figure out how to fix the build fail on this -- fails with TimeoutError: Timed out after 30000 ms while trying to connect to Chrome, maybe linked to running puppeteer twice in a row (webapp, then bext)?