patch 8.1.2218: "gN" is off by one in Visual mode

Problem:    "gN" is off by one in Visual mode.
Solution:   Check moving forward. (Christian Brabandt, #5075)
diff --git a/src/testdir/test_gn.vim b/src/testdir/test_gn.vim
index ecf9b82..9edf0a2 100644
--- a/src/testdir/test_gn.vim
+++ b/src/testdir/test_gn.vim
@@ -148,6 +148,11 @@
   norm! gg0f7vhhhhgnd
   call assert_equal(['12348'], getline(1,'$'))
   sil! %d _
+  call setline('.', ['12345678'])
+  let @/ = '5'
+  norm! gg0f2vf7gNd
+  call assert_equal(['1678'], getline(1,'$'))
+  sil! %d _
 
   set wrapscan&vim
 endfu