fix(test): Close all windows but the first after a test run. (#4897) r=@philbooth
fix(test): Close all windows but the first after a test run. (#4897) r=@philbooth

If a test opens a new tab and checks for an element in the new
tab, but fails, then the 2nd tab is never closed. This causes
cascading failures down the line.

In `clearBrowserState`, close all windows but the first.

fixes #4896
mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server