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/phpunit/classes/util.php

@@ -208,6 +208,7 @@ class phpunit_util extends testing_util {
         // reset all static caches
         \core\event\manager::phpunit_reset();
         accesslib_clear_all_caches(true);
+        accesslib_reset_role_cache();
         get_string_manager()->reset_caches(true);
         reset_text_filters_cache(true);
         core_text::reset_caches();