Suppress a safemutex warning pending fix of MBug#578117.

mysql-test/suite/pbxt/r/pbxt_xa.result

 drop table if exists t1, t2;
+CALL mtr.add_suppression("Found wrong usage of mutex 'LOCK_sync' and 'LOCK_active'");
 CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=innodb;
 CREATE TABLE t2 (b INT PRIMARY KEY) ENGINE=pbxt;
 BEGIN;

mysql-test/suite/pbxt/t/pbxt_xa.test

@@ -4,6 +4,11 @@
 drop table if exists t1, t2;
 --enable_warnings
 
+# This warning is indication of a real bug, MBug#578117.
+# But it is not a regression, so we suppress it to get a clean test run.
+# This suppression must be removed as part of MBug#578117 fix.
+CALL mtr.add_suppression("Found wrong usage of mutex 'LOCK_sync' and 'LOCK_active'");
+
 #
 # bug lp:544173, xa crash with two 2pc-capable storage engines without binlog
 #