tests: use i3_config arg instead of precisely one launch_with_config

This way, tests are shorter, and i3test’s invocation of launch_with_config
parallelizes work better, using dont_block => 1.

testcases/t/500-multi-monitor.t

@@ -17,15 +17,12 @@
 # Tests that the provided X-Server to the t/5??-*.t tests is actually providing
 # multiple monitors.
 #
-use i3test i3_autostart => 0;
-
-my $config = <<EOT;
+use i3test i3_config => <<EOT;
 # i3 config file (v4)
 font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
 
 fake-outputs 1024x768+0+0,1024x768+1024+0
 EOT
-my $pid = launch_with_config($config);
 
 my $i3 = i3(get_socket_path());
 
@@ -39,6 +36,5 @@ my @outputs = map { $_->{name} } @{$tree->{nodes}};
 is_deeply(\@outputs, [ '__i3', 'fake-0', 'fake-1' ],
           'multi-monitor outputs ok');
 
-exit_gracefully($pid);
 
 done_testing;