patch 9.1.0205: Cannot use modifiers before :-Ntabmove

Problem:  Cannot use modifiers before :-Ntabmove.
Solution: Check backwards from the command instead of checking from the
          start of the command line. Slightly adjust docs to make them
          more consistent (zeertzjq).

closes: #14289

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_tabpage.vim b/src/testdir/test_tabpage.vim
index a531f19..3624790 100644
--- a/src/testdir/test_tabpage.vim
+++ b/src/testdir/test_tabpage.vim
@@ -117,10 +117,16 @@
   call assert_equal(3, tabpagenr())
   +3tabmove
   call assert_equal(6, tabpagenr())
+  silent -tabmove
+  call assert_equal(5, tabpagenr())
+  silent -2 tabmove
+  call assert_equal(3, tabpagenr())
+  silent	-2	tabmove
+  call assert_equal(1, tabpagenr())
 
-  " The following are a no-op
   norm! 2gt
   call assert_equal(2, tabpagenr())
+  " The following are a no-op
   tabmove 2
   call assert_equal(2, tabpagenr())
   2tabmove