Fix galera.galera_suspend_slave on FreeBSD

Use symbolic signal names (e.g. SIGSTOP) instead of numeric ones (e.g.
19) because the latter are not portable.

mysql-test/suite/galera/t/galera_suspend_slave.test

@@ -25,7 +25,7 @@ CREATE TABLE t1 (f1 INTEGER PRIMARY KEY) ENGINE=InnoDB;
 	my $pid_filename = $ENV{'NODE_2_PIDFILE'};
 	my $mysqld_pid = `cat $pid_filename`;
         chomp($mysqld_pid);
-	system("kill -19 $mysqld_pid");
+	system("kill -SIGSTOP $mysqld_pid");
 	exit(0);
 EOF
 
@@ -37,7 +37,7 @@ INSERT INTO t1 VALUES (1);
 	my $pid_filename = $ENV{'NODE_2_PIDFILE'};
 	my $mysqld_pid = `cat $pid_filename`;
         chomp($mysqld_pid);
-	system("kill -18 $mysqld_pid");
+	system("kill -SIGCONT $mysqld_pid");
 	exit(0);
 EOF