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.

src/option.c

@@ -4330,7 +4330,8 @@ find_key_option(char_u *arg_arg, int has_lt)
     {
 	--arg;			    // put arg at the '<'
 	modifiers = 0;
-	key = find_special_key(&arg, &modifiers, TRUE, TRUE, FALSE, TRUE, NULL);
+	key = find_special_key(&arg, &modifiers,
+			    FSK_KEYCODE | FSK_KEEP_X_KEY | FSK_SIMPLIFY, NULL);
 	if (modifiers)		    // can't handle modifiers here
 	    key = 0;
     }