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/518-interpret-workspace-numbers.t

@@ -18,9 +18,7 @@
 # assign any workspace of that number to the specified output.
 # Ticket: #1238
 # Bug still in: 4.7.2-147-g3760a48
-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
 
@@ -32,7 +30,6 @@ workspace 2:override output fake-1
 fake-outputs 1024x768+0+0,1024x768+1024+0
 EOT
 
-my $pid = launch_with_config($config);
 my $i3 = i3(get_socket_path());
 $i3->connect->recv;
 
@@ -72,6 +69,4 @@ is(get_output_for_workspace('1:override'), 'fake-0',
     'Assignment rules should not be affected by the order assignments are declared')
     or diag 'Since workspace "1:override" is assigned by name to fake-0, it should open on fake-0';
 
-exit_gracefully($pid);
-
 done_testing;