Allow external test runners to integrate more tightly with Saga

This change allows external test runners to integrate more tightly with Saga by providing the IntrumentingBrowser instance to be used for running the tests via the config.  This allows the external test runner to report the test run results when the tests have completed.

This change adds the new config methods set/getInstrumentingBrowser().  If an InstrumentingBrowser instance is provided, then Saga will use it instead of creating its own.  When the test run has completed, the browser will be left open so that the test results can be processed by the external test runner.