test: make expect_unchanged() less confusing (#22255)

Problem:
The sleep before collecting the initial screen state is confusing and
may lead to unexpected success if it comes after a blocking RPC call.

Solution:
Remove that sleep and add an "intermediate" argument.

test/functional/ui/inccommand_spec.lua

@@ -2886,7 +2886,8 @@ it(':substitute with inccommand during :terminal activity', function()
     feed('gg')
     feed(':%s/foo/ZZZ')
     sleep(20)  -- Allow some terminal activity.
-    helpers.poke_eventloop()
+    poke_eventloop()
+    screen:sleep(0)
     screen:expect_unchanged()
   end)
 end)