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/244-new-workspace-floating-enable-center.t

@@ -17,9 +17,7 @@
 # Ensures that 'move workspace $new, floating enable' on a marked window
 # leaves the window centered on the new workspace.
 # Bug still in: 4.10.2-137-ga4f0ed6
-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
 
@@ -27,8 +25,6 @@ new_window none
 new_float none
 EOT
 
-my $pid = launch_with_config($config);
-
 #####################################################################
 # Open a tiled window, and then simultaneously move it to another
 # workspace and float it, ensuring that it ends up centered.
@@ -48,6 +44,4 @@ is(int($pos->{x} + $pos->{width} / 2), int($x->root->rect->width / 2),
 is(int($pos->{y} + $pos->{height} / 2), int($x->root->rect->height / 2),
     'y coordinates match');
 
-exit_gracefully($pid);
-
 done_testing;