reenable browser extension e2e tests
Created by: sqs
The build steps for the browser extension e2e tests were commented out in 1567da237f37824fad6238e6388fb0fca233d76e (2018 Nov 20) by @ijsnow. This reenables them (and makes it possible to run them by pushing to bext/e2e
without releasing).
@ijsnow Why did you disable these? They appear to pass (although after the tests pass, the process hangs in CI, possibly due to some other issue). See https://buildkite.com/sourcegraph/sourcegraph/builds/25662#800d7c44-72a9-422b-a73b-4fb5c703dc79 for an example of the hang.