test/old: skip Test_swapfile_delete() until "blob" is merged

src/nvim/testdir/test_swap.vim

@@ -127,6 +127,7 @@ func Test_swapname()
 endfunc
 
 func Test_swapfile_delete()
+  throw 'skipped: need the "blob" feature for this test'
   autocmd! SwapExists
   function s:swap_exists()
     let v:swapchoice = s:swap_choice