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/237-regress-assign-focus.t

@@ -18,9 +18,7 @@
 # assigned to an invisible workspace) will not crash i3.
 # Ticket: #1338
 # Bug still in: 4.8-91-g294d52e
-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 [title=".*"] 1
 for_window [title=".*"] layout tabbed, focus
 EOT
 
-my $pid = launch_with_config($config);
-
 # Switch away from workspace 1
 my $tmp = fresh_workspace;
 
@@ -37,6 +33,4 @@ my $win = open_window;
 
 does_i3_live;
 
-exit_gracefully($pid);
-
 done_testing;