Minimal supporting for Selenium browser tests

Like https://gerrit.wikimedia.org/r/#/c/88702/ for the Translate
extension, this is the minimum set of files to have browsertests for
Flow in the extension rather than qa/browsertests.git

The actual Flow tests follow in a separate commit.

Change-Id: Ib85f50fc3bc18a82635f48be7ff6c3ec8513e52c