MDEV-19423: Galera test failure on galera.MDEV-16509

Make sure that SIGNALs are not overwritten before they
are received.

mysql-test/suite/galera/r/MDEV-16509.result

@@ -63,8 +63,9 @@ SET DEBUG_SYNC = "now WAIT_FOR agac_reached_2";
 wsrep_last_seen_gtid_no_match
 1
 SET DEBUG_SYNC = "now SIGNAL agac_continue_1";
-SET DEBUG_SYNC = "now SIGNAL agac_continue_2";
 connection node_1;
+connection ctrl;
+SET DEBUG_SYNC = "now SIGNAL agac_continue_2";
 connection node_1a;
 connection ctrl;
 SET DEBUG_SYNC = "RESET";

mysql-test/suite/galera/t/MDEV-16509.test

@@ -124,10 +124,13 @@ SET DEBUG_SYNC = "now WAIT_FOR agac_reached_2";
 --enable_query_log
 
 SET DEBUG_SYNC = "now SIGNAL agac_continue_1";
-SET DEBUG_SYNC = "now SIGNAL agac_continue_2";
 
 --connection node_1
 --reap
+
+--connection ctrl
+SET DEBUG_SYNC = "now SIGNAL agac_continue_2";
+
 --connection node_1a
 --reap