Temporarily disable a failing test

On several buildbot machines, the test fails like this:

CURRENT_TEST: wsrep.binlog_format
mysqltest: At line 12: query 'SET binlog_format=STATEMENT' failed:
1231: Variable 'binlog_format' can't be set to the value of 'STATEMENT'

mysql-test/suite/wsrep/disabled.def

 foreign_key : MDEV-7915
+binlog_format : MDEV-11532