patch 8.2.0293: various Ex commands not sufficiently tested

Problem:    Various Ex commands not sufficiently tested.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes #5673)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 276719c..2b51b57 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -484,6 +484,7 @@
 		\ 'RegularLine2']
 
   Xfirst
+  call assert_fails('-5Xcc', 'E16:')
   call assert_fails('Xprev', 'E553')
   call assert_fails('Xpfile', 'E553')
   Xnfile