Fix test failure on galera-features#56

We need to use wsrep_sync_wait=15 before selects to make sure
that node has applied all inserts from other node.

mysql-test/suite/galera_sr/r/galera-features#56.result

@@ -26,6 +26,7 @@ INSERT INTO t1 (f2) SELECT 1 FROM ten AS a1, ten AS a2, ten AS a3, ten AS a4;;
 connection node_1;
 connection node_1a;
 connection node_2;
+set session wsrep_sync_wait=15;
 SELECT COUNT(*) FROM t1;
 COUNT(*)
 30000

mysql-test/suite/galera_sr/t/galera-features#56.test

@@ -52,6 +52,7 @@ SET SESSION wsrep_trx_fragment_size = 1;
 --connection node_2
 --reap
 
+set session wsrep_sync_wait=15;
 SELECT COUNT(*) FROM t1;
 SELECT COUNT(DISTINCT f1) FROM t1;