Bug 968709 - Add a marionette test case to Bluetooth set to make sure the URL of Marionette client is correct. r=echou, f=vyang

Few marionette tests in unit-test set may navigate to specific URL and
wouldn't restore the URL of marionette client when the tests end.
Navigate the URL to default value before running Bluetooth test to make
sure the correctness of test.
Bug 968709 - Add a marionette test for pairing APIs of BluetoothAdapter. r=echou, f=vyang