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/199-ipc-mode-event.t

@@ -15,9 +15,7 @@
 #   (unless you are already familiar with Perl)
 #
 # Verifies that the IPC 'mode' event is sent when modes are changed.
-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
 
@@ -30,8 +28,6 @@ mode "with spaces" {
 }
 EOT
 
-my $pid = launch_with_config($config);
-
 my $i3 = i3(get_socket_path(0));
 $i3->connect->recv;
 
@@ -52,6 +48,4 @@ $t = AnyEvent->timer(after => 0.5, cb => sub { $cv->send(0); });
 
 ok($cv->recv, 'Mode event received');
 
-exit_gracefully($pid);
-
 done_testing;