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_backspace_opt.vim b/src/testdir/test_backspace_opt.vim
index 503643a..149d8a8 100644
--- a/src/testdir/test_backspace_opt.vim
+++ b/src/testdir/test_backspace_opt.vim
@@ -86,7 +86,7 @@
 
   set cpo-=<
   inoremap <c-u> <left><c-u>
-  exe "normal Avim3\{C-U}\<Esc>\<CR>"
+  exe "normal Avim3\<*C-U>\<Esc>\<CR>"
   iunmap <c-u>
   exe "normal Avim4\<C-U>\<C-U>\<Esc>\<CR>"
 
@@ -96,7 +96,7 @@
   exe "normal A vim6\<Esc>Azwei\<C-G>u\<C-U>\<Esc>\<CR>"
 
   inoremap <c-u> <left><c-u>
-  exe "normal A vim7\{C-U}\{C-U}\<Esc>\<CR>"
+  exe "normal A vim7\<*C-U>\<*C-U>\<Esc>\<CR>"
 
   call assert_equal([
         \ "1 this shouldn't be deleted",