Disable galera_var_notify_cmd as it causes hang.

Author: Jan Lindström

mysql-test/suite/galera/disabled.def

@@ -43,3 +43,4 @@ galera_wan : MDEV-17259 Test failure on galera.galera_wan
 mysql-wsrep#198 : MDEV-18935 Galera test mysql-wsrep#198 sporaric assertion transaction.cpp:362: int wsrep::transaction::before_commit(): Assertion `state() == s_executing || state() == s_committing || state() == s_must_abort || state() == s_replaying' failed.
 partition : MDEV-19958 Galera test failure on galera.partition
 query_cache: MDEV-15805 Test failure on galera.query_cache
+galera_var_notify_cmd : MDEV-20600 Galera test galera_var_notify_cmd causes hang