Stabilize rocksdb_rpl.multiclient_2pc (facebook#787) (facebook#787)

Summary:
The test fails sporadically because the statement to signal a crash doesn't always return before the crash actually happens, depending how the threads are scheduled.

Closes facebook#716
Closes facebook#787

Differential Revision: D6740009 (facebook@2f0b8f5)

Pulled By: Tema

fbshipit-source-id: a0283169857

mysql-test/suite/rocksdb_rpl/t/multiclient_2pc.test

@@ -62,6 +62,7 @@ SET GLOBAL SYNC_BINLOG = 1;
 
 insert into t1 values (1000000, 1, "i_am_just_here_to_trigger_a_flush");
 
+--error 0,2013
 SET DEBUG_SYNC='now SIGNAL go';
 --source include/wait_until_disconnected.inc
 --enable_reconnect