t/158-wm_take_focus: use sync_with_i3 instead of a timeout

[i3/i3] / GOALS

GOALS (these have to be fullfilled all the time):

 * Do _NOT_ use autotools or a similar mechanism. Instead, document all
   requirements properly so that packagers for every plattform can easily
   provide users with a package. Those who build software for themselves
   do know how to install requirements, if they are documented.