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/208-regress-floating-criteria.t

@@ -18,9 +18,7 @@
 # criteria for windows on non-visible workspaces.
 # Ticket: #1027
 # Bug still in: 4.5.1-90-g6582da9
-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
 
@@ -28,8 +26,6 @@ assign [class="^special$"] → mail
 for_window [class="^special$"] floating enable, floating disable
 EOT
 
-my $pid = launch_with_config($config);
-
 my $window = open_window(
     name => 'Borderless window',
     wm_class => 'special',
@@ -43,6 +39,4 @@ cmd '[class="^special$"] focus';
 
 does_i3_live;
 
-exit_gracefully($pid);
-
 done_testing;