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/233-regress-manage-focus-unmapped.t

@@ -19,9 +19,7 @@
 # which can lead to complications
 # Ticket: #1283
 # Bug still in: 4.8-24-g60070de
-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
 
@@ -29,8 +27,6 @@ for_window [class="^special_kill$"] kill
 for_window [class="^special_scratchpad$"] move scratchpad
 EOT
 
-my $pid = launch_with_config($config);
-
 my $win = open_window;
 
 my $scratch_window = open_window(
@@ -53,6 +49,4 @@ sync_with_i3;
 is($x->input_focus, $win->{id},
     'an assignment that kills a window should not disturb focus');
 
-exit_gracefully($pid);
-
 done_testing;