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/searchhl_spec.lua

@@ -317,6 +317,7 @@ describe('search highlighting', function()
     ]])
     feed('/foo')
     helpers.poke_eventloop()
+    screen:sleep(0)
     screen:expect_unchanged()
   end)