test-current-desktop: add extra step when killing a client

Ref: https://github.com/awesomeWM/awesome/issues/1495#issuecomment-277098634

tests/test-current-desktop.lua

@@ -72,10 +72,13 @@ local steps = {
     wait_for_current_desktop(tags[4]),
 
     -- Killing the client means the first selected tag counts
-    function()
+    function(count)
+        if count == 1 then
             assert(client.focus == c)
             c:kill()
             c = nil
+            return
+        end
         return true
     end,
     wait_for_current_desktop(tags[3]),