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/fold_spec.lua

@@ -10,7 +10,6 @@ local meths = helpers.meths
 local exec = helpers.exec
 local exec_lua = helpers.exec_lua
 local assert_alive = helpers.assert_alive
-local poke_eventloop = helpers.poke_eventloop
 
 
 local content1 = [[
@@ -30,8 +29,6 @@ describe("folded lines", function()
   local function with_ext_multigrid(multigrid)
     local screen
     before_each(function()
-      clear()
-      command('hi VertSplit gui=reverse')
       screen = Screen.new(45, 8)
       screen:attach({rgb=true, ext_multigrid=multigrid})
       screen:set_default_attr_ids({
@@ -166,12 +163,10 @@ describe("folded lines", function()
       end
       -- CursorLine is applied correctly with screenrow motions #22232
       feed("jgk")
-      poke_eventloop()
-      screen:expect_unchanged()
+      screen:expect_unchanged(true)
       -- CursorLine is applied correctly when closing a fold when cursor is not at fold start
       feed("zo4Gzc")
-      poke_eventloop()
-      screen:expect_unchanged()
+      screen:expect_unchanged(true)
       command("set cursorlineopt=line")
       if multigrid then
         screen:expect([[

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)

test/functional/ui/screen.lua

@@ -470,15 +470,19 @@ screen:redraw_debug() to show all intermediate screen states.  ]])
   end, expected)
 end
 
-function Screen:expect_unchanged(waittime_ms, ignore_attrs, request_cb)
+function Screen:expect_unchanged(intermediate, waittime_ms, ignore_attrs)
   waittime_ms = waittime_ms and waittime_ms or 100
   -- Collect the current screen state.
-  self:sleep(0, request_cb)
   local kwargs = self:get_snapshot(nil, ignore_attrs)
 
-  -- Check that screen state does not change.
-  kwargs.unchanged = true
+  if intermediate then
+    kwargs.intermediate = true
+  else
+    kwargs.unchanged = true
+  end
+
   kwargs.timeout = waittime_ms
+  -- Check that screen state does not change.
   self:expect(kwargs)
 end
 

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)