Disable a badly written, randomly failing Galera test

CURRENT_TEST: galera.galera_kill_applier
mysqltest: At line 14: query 'KILL $applier_thread' failed with wrong
errno 1064: 'You have an error in your SQL syntax; check the manual
that corresponds to your MariaDB server version for the right syntax
to use near '' at line 1', instead of 1095...

mysql-test/suite/galera/disabled.def

@@ -55,3 +55,4 @@ galera_lock_table : MDEV-13549 Galera test failures 10.1
 MW-284 : MDEV-13549 Galera test failures 10.1
 galera_as_slave : MDEV-13549 Galera test failures 10.1
 galera_var_innodb_disallow_writes : MDEV-10949
+galera_kill_applier : race condition at the start of the test