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_backspace_opt.vim b/src/testdir/test_backspace_opt.vim
index ad051cc..503643a 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",