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_mapping.vim b/src/testdir/test_mapping.vim
index f6e7124..7eb6e34 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -76,7 +76,7 @@
   inoremap <c-c> <ctrl-c>
   cnoremap <c-c> dummy
   cunmap <c-c>
-  call feedkeys("GoTEST2: CTRL-C |\<C-C>A|\<Esc>", "xt")
+  call feedkeys("GoTEST2: CTRL-C |\{C-C}A|\<Esc>", "xt")
   call assert_equal('TEST2: CTRL-C |<ctrl-c>A|', getline('$'))
   unmap! <c-c>
   set nomodified
@@ -85,7 +85,7 @@
 func Test_map_ctrl_c_visual()
   " mapping of ctrl-c in Visual mode
   vnoremap <c-c> :<C-u>$put ='vmap works'
-  call feedkeys("GV\<C-C>\<CR>", "xt")
+  call feedkeys("GV\{C-C}\<CR>", "xt")
   call assert_equal('vmap works', getline('$'))
   vunmap <c-c>
   set nomodified
@@ -235,7 +235,7 @@
 
 func Test_map_meta_quotes()
   imap <M-"> foo
-  call feedkeys("Go-\<M-\">-\<Esc>", "xt")
+  call feedkeys("Go-\{M-\"}-\<Esc>", "xt")
   call assert_equal("-foo-", getline('$'))
   set nomodified
   iunmap <M-">