Restarting B2G before running tests, unlock screen improvements

Haha, I agree, but I may have a solution in place in `marionette-device-host`. The goal is to restart B2G in-between each set of tests per app, but not each test per app. For example, if we are running `make test-perf` for Settings and Gallery, we want to restart B2G once before all the Settings tests, and once again before all the Gallery tests.