MDL-69232 behat: Make selenium start more fault tolerant
The session was previously only started once, but has now been moved to restart between each test. Because of this the `$session->start()` is now called more frequently which exposes a pre-existing issue whereby Selenium will periodically fail to start. When the session was only started at the beginning of the test, and at occasional other times, it made sense to always stop the test runner because it signified an issue with obtaining the session at all. Since the session is restarted more frequently, and now between every test, it no longer makes sense to do this. The test should only be _stopped_ if the first attempt to obtain the Session fails. Once a Session has been retrieved at least once it no longer makes sense to assume that the failure was a configuration failure, but a connection failure. Therefore subsequent tests should be attempted.