Testcases: init pointer in a predictable position

Long description:

Tests may disturb the pointer in their normal operation that may lead to
unexpected results in later tests using that display. Reset the pointer
before a test begins to (0, 0) to save test developers from related
"gotchas" and reduce multi-monitor test boilerplate.

To apply this patch, use:
curl http://cr.i3wm.org/patch/341/raw.patch | git am

@@ -155,6 +155,9 @@ __

     warnings->import;

     $x ||= i3test::X11->new;

+    # set the pointer to a predictable position in case a previous test has

+    # disturbed it

+    $x->root->warp_pointer(0, 0);

     $cv->recv if $i3_autostart;

     @_ = ($class);