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/typval.c b/src/typval.c
index cd2a862..95b2792 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -1285,15 +1285,24 @@
 			  ++name;
 			  break;
 
-			    // Special key, e.g.: "\<C-W>"
-		case '<': extra = trans_special(&p, name, TRUE, TRUE,
-								   TRUE, NULL);
-			  if (extra != 0)
+			  // Special key, e.g.: "\<C-W>" or "\{C-W}"
+		case '<':
+		case '{':
 			  {
-			      name += extra;
-			      if (name >= rettv->vval.v_string + len)
-				  iemsg("get_string_tv() used more space than allocated");
-			      break;
+			      int flags = FSK_KEYCODE | FSK_IN_STRING;
+
+			      if (*p == '<')
+				  flags |= FSK_SIMPLIFY;
+			      else
+				  flags |= FSK_CURLY;
+			      extra = trans_special(&p, name, flags, NULL);
+			      if (extra != 0)
+			      {
+				  name += extra;
+				  if (name >= rettv->vval.v_string + len)
+				      iemsg("get_string_tv() used more space than allocated");
+				  break;
+			      }
 			  }
 			  // FALLTHROUGH