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/156-fullscreen-focus.t

@@ -18,22 +18,16 @@
 # the time of launching the new one. Also make sure that focusing containers
 # in other workspaces work even when there is a fullscreen container.
 #
-use i3test i3_autostart => 0;
-
-# Screen setup looks like this:
-# +----+----+
-# | S1 | S2 |
-# +----+----+
-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());
+# Screen setup looks like this:
+# +----+----+
+# | S1 | S2 |
+# +----+----+
 
 my $tmp = fresh_workspace;
 
@@ -331,6 +325,4 @@ verify_move(2, 'prevented move to workspace by position');
 
 # TODO: Tests for "move to output" and "move workspace to output".
 
-exit_gracefully($pid);
-
 done_testing;