patch 8.1.1538: cannot specify highlighting for notifications

Problem:    Cannot specify highlighting for notifications.
Solution:   Use the PopupNotification group if it exists. Add a minimal width
            to notifications.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 789c859..d9b0b29 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1191,7 +1191,8 @@
   call VerifyScreenDump(buf, 'Test_popupwin_notify_01', {})
 
   " second one goes below the first one
-  call term_sendkeys(buf, ":call popup_notification('another important notification', {'highlight': 'Notification'})\<CR>")
+  call term_sendkeys(buf, ":hi link PopupNotification Notification\<CR>")
+  call term_sendkeys(buf, ":call popup_notification('another important notification', {})\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_notify_02', {})