tests: Make i3 create its temporary files in /tmp/i3-testsuite

This behavior can be avoided by passing dont_create_temp_dir => 1 to
launch_with_config (or activate_i3).

This commit fixes t/159-socketpaths.t being flaky on non-systemd computers.

testcases/lib/SocketActivation.pm

@@ -52,6 +52,10 @@ sub activate_i3 {
         $ENV{LISTEN_PID} = $$;
         $ENV{LISTEN_FDS} = 1;
         delete $ENV{DESKTOP_STARTUP_ID};
+        unless ($args{dont_create_temp_dir}) {
+            $ENV{XDG_RUNTIME_DIR} = '/tmp/i3-testsuite/';
+            mkdir $ENV{XDG_RUNTIME_DIR};
+        }
         $ENV{DISPLAY} = $args{display};
         $ENV{PATH} = join(':',
             '../i3-nagbar',