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/lib/SocketActivation.pm

@@ -49,6 +49,11 @@ sub activate_i3 {
         die "could not fork()";
     }
     if ($pid == 0) {
+        # Start a process group so that in the parent, we can kill the entire
+        # process group and immediately kill i3bar and any other child
+        # processes.
+        setpgrp;
+
         $ENV{LISTEN_PID} = $$;
         $ENV{LISTEN_FDS} = 1;
         delete $ENV{DESKTOP_STARTUP_ID};

testcases/lib/i3test.pm.in

@@ -126,7 +126,7 @@ END {
         exit_gracefully($i3_pid, "/tmp/nested-$ENV{DISPLAY}");
 
     } else {
-        kill(9, $i3_pid)
+        kill(-9, $i3_pid)
             or $tester->BAIL_OUT("could not kill i3");
 
         waitpid $i3_pid, 0;
@@ -138,8 +138,9 @@ sub import {
     my $pkg = caller;
 
     $i3_autostart = delete($args{i3_autostart}) // 1;
+    my $i3_config = delete($args{i3_config}) // '-default';
 
-    my $cv = launch_with_config('-default', dont_block => 1)
+    my $cv = launch_with_config($i3_config, dont_block => 1)
         if $i3_autostart;
 
     my $test_more_args = '';