MDL-54628 behat: Set behat dir for each run under behat_dataroot
Before this patch behat_dataroot for parallel runs were created at same level as ->behat_dataroot After this patch, it will be created 1 level under ->behat_dataroot with BEHAT_PARALLEL_SITE_NAME prefix and run number as suffix. This will allow common files as test enabled and parallel run info to be saved in parent directory and access it easily.