make mtr_check to monitor mysql.plugin table too.

fix tests to clean up properly

mysql-test/include/mtr_check.sql

@@ -67,6 +67,7 @@ BEGIN
     mysql.help_keyword,
     mysql.help_relation,
     mysql.host,
+    mysql.plugin,
     mysql.proc,
     mysql.procs_priv,
     mysql.roles_mapping,