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/521-ewmh-desktop-viewport.t

@@ -18,9 +18,7 @@
 # properly on the root window. We interpret this as a list of x/y coordinate
 # pairs for the upper left corner of the respective outputs of the workspaces
 # Ticket: #1241
-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 @@ workspace 1 output fake-1
 fake-outputs 1024x768+0+0,1024x768+1024+0
 EOT
 
-my $pid = launch_with_config($config);
-
 sub get_desktop_viewport {
     # Make sure that i3 pushed its changes to X11 before querying.
     sync_with_i3;
@@ -90,6 +86,4 @@ cmd 'workspace 0';
 is_deeply(\@desktop_viewport, \@expected_viewport,
     'it should be updated when a workspace is emptied');
 
-exit_gracefully($pid);
-
 done_testing;