patch 9.0.0683: cannot specify a time for :echowindow

Problem:    Cannot specify a time for :echowindow.
Solution:   A count can be used to specify the display time. Add
            popup_findecho().
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 24995f1..ab33caa 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -511,6 +511,10 @@
         echo 'two'
         echo 'three'
       enddef
+
+      def HideWin()
+        popup_hide(popup_findecho())
+      enddef
   END
   call writefile(lines, 'XtestEchowindow', 'D')
   let buf = RunVimInTerminal('-S XtestEchowindow', #{rows: 8})
@@ -536,9 +540,12 @@
   call VerifyScreenDump(buf, 'Test_echowindow_7', {})
 
   call term_sendkeys(buf, ":tabnew\<CR>")
-  call term_sendkeys(buf, ":echowin 'more'\<CR>")
+  call term_sendkeys(buf, ":7echowin 'more'\<CR>")
   call VerifyScreenDump(buf, 'Test_echowindow_8', {})
 
+  call term_sendkeys(buf, ":call HideWin()\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_9', {})
+
   " clean up
   call StopVimInTerminal(buf)
 endfunc