MDL-54035 accesslib: separate role definition cache clear

Thanks to MDL-49398, we can separate the combined user session and role
definition cache clearing function into two separate functions. At the
same time, we want to identify and remove mark_dirty() calls that were
added for role definition changes but were incorrectly left behind.

Change highlights:
 - Remove unnecessary mark_dirty() calls performed after
   assign_capability(), unassign_capability(), delete_role(),
   deleted contexts, brand new contexts
 - Move role definition cache clear from the user-centric
   accesslib_clear_all_caches() to the newly created,
   role-dedicated accesslib_reset_role_cache()

lib/behat/classes/util.php

@@ -354,6 +354,7 @@ class behat_util extends testing_util {
 
         // Reset all static caches.
         accesslib_clear_all_caches(true);
+        accesslib_reset_role_cache();
         // Reset the nasty strings list used during the last test.
         nasty_strings::reset_used_strings();