patch 9.0.0865: duplicate arguments are not always detected

Problem:    Duplicate arguments are not always detected.
Solution:   Expand to full path before comparing arguments. (Nir Lichtman,
            closes #11505, closes #9402)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index 1bc556f..edc8b77 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -420,15 +420,19 @@
   call Reset_arglist()
   argdedupe
   call assert_equal([], argv())
+
   args a a a aa b b a b aa
   argdedupe
   call assert_equal(['a', 'aa', 'b'], argv())
+
   args a b c
   argdedupe
   call assert_equal(['a', 'b', 'c'], argv())
+
   args a
   argdedupe
   call assert_equal(['a'], argv())
+
   args a A b B
   argdedupe
   if has('fname_case')
@@ -436,11 +440,17 @@
   else
     call assert_equal(['a', 'b'], argv())
   endif
+
   args a b a c a b
   last
   argdedupe
   next
   call assert_equal('c', expand('%:t'))
+
+  args a ./a
+  argdedupe
+  call assert_equal(['a'], argv())
+
   %argd
 endfunc