patch 8.2.0855: GUI tests fail because the test doesn't use a modifier

Problem:    GUI tests fail because the test doesn't use a modifier.
Solution:   Add "\{xxx}" to be able to encode a modifier.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index b74ad2c..832762a 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -309,7 +309,7 @@
 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