patch 8.1.1604: popup window scroll test is flaky

Problem:    Popup window scroll test is flaky.
Solution:   Add a delay between scroll events.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index e80621b..1cb0734 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1459,6 +1459,8 @@
   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', {})