patch 8.1.2165: mapping test fails on Mac

Problem:    Mapping test fails on Mac.
Solution:   Remove the default Mac mapping.
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index f62d575..d3ae615 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -444,6 +444,9 @@
 endfunc
 
 func Test_list_mappings()
+  " Remove default Mac mapping
+  silent! iunmap <D-v>
+
   inoremap <C-M> CtrlM
   inoremap <A-S> AltS
   inoremap <S-/> ShiftSlash