patch 8.2.0867: using \{xxx} for encoding a modifier is not nice

Problem:    Using \{xxx} for encoding a modifier is not nice.
Solution:   Use \<*xxx> instead, since it's the same as \<xxx> but producing a
            different code.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 5d6959b..f26b8c0 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -306,7 +306,7 @@
 func Test_mapping_at_hit_return_prompt()
   nnoremap <C-B> :echo "hit ctrl-b"<CR>
   call feedkeys(":ls\<CR>", "xt")
-  call feedkeys("\{C-B}", "xt")
+  call feedkeys("\<*C-B>", "xt")
   call assert_match('hit ctrl-b', Screenline(&lines - 1))
   nunmap <C-B>
 endfunc