patch 8.1.1604: popup window scroll test is flaky

Problem:    Popup window scroll test is flaky.
Solution:   Add a delay between scroll events.

Author: brammool

src/testdir/test_popupwin.vim

@@ -1459,6 +1459,8 @@ func Test_popup_scrollbar()
   call VerifyScreenDump(buf, 'Test_popupwin_scroll_6', {})
 
   call term_sendkeys(buf, ":call ScrollDown()\<CR>")
+  " wait a bit, otherwise it fails sometimes (double click recognized?)
+  sleep 100m
   call term_sendkeys(buf, ":call ScrollDown()\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_scroll_7', {})
 

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1604,
 /**/
     1603,
 /**/