testcases: always start i3 through i3test::launch_with_config

this will pave the way to just call BAIL_OUT and stop the currently
running testfile if i3 died for whatever reason.

testcases/t/165-for_window.t

@@ -1,9 +1,7 @@
 #!perl
 # vim:ts=4:sw=4:expandtab
-# !NO_I3_INSTANCE! will prevent complete-run.pl from starting i3
 #
-#
-use i3test;
+use i3test i3_autostart => 0;
 use X11::XCB qw(PROP_MODE_REPLACE);
 
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