Tie browsertests jobs to wmflabs slaves

We need to have bundler and internet access to fetch ruby gems, so tie
the browsertests jobs to the wmflabs slaves integration-slave1001 and
integration-slave1002 which have the 'hasBrowserTests' label.

Paired with Željko Filipin.

Bug: 58040
Change-Id: I076a0c027e776866e4a4da1ba8a96ad99c7f0bcd