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.

lib/phpunit/classes/util.php

@@ -412,10 +412,12 @@ class phpunit_util extends testing_util {
 
         self::reset_dataroot();
         testing_initdataroot($CFG->dataroot, 'phpunit');
-        self::drop_dataroot();
 
-        // drop all tables
+        // Drop all tables.
         self::drop_database($displayprogress);
+
+        // Drop dataroot.
+        self::drop_dataroot();
     }
 
     /**