Retry browser tests when they report a conection failure

[JeremyRand]

The browser tests (Chromium/Firefox) seem to be very sensitive to network issues, and will often report a connection failure; this causes spurious Cirrus failures for us. It would be desirable to retry those tests a few times if a connection failure happens. Preferably do not retry if a certificate error is reported (not sure how easy it is to disambiguate the two).