Increase default MTR_PORT_GROUP_SIZE to 20.

Some Galera tests can require more than 10 ports.

mysql-test/mysql-test-run.pl

@@ -363,7 +363,7 @@ my $opt_max_save_datadir= env_or_val(MTR_MAX_SAVE_DATADIR => 20);
 my $opt_max_test_fail= env_or_val(MTR_MAX_TEST_FAIL => 10);
 
 my $opt_parallel= $ENV{MTR_PARALLEL} || 1;
-my $opt_port_group_size = $ENV{MTR_PORT_GROUP_SIZE} || 10;
+my $opt_port_group_size = $ENV{MTR_PORT_GROUP_SIZE} || 20;
 
 # lock file to stop tests
 my $opt_stop_file= $ENV{MTR_STOP_FILE};