test-geometry: Wait for c:kill() to be done

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/test-geometry.lua

@@ -65,7 +65,10 @@ local steps = {
                 height            = 100
             }}
         }
-        return true
+        -- Wait for the previous c:kill() to be done
+        if #client.get() == 0 then
+            return true
+        end
     end,
     function(count)
         if count == 1 then