test(str): incorrect group specification (#567)

Author: rgrinberg

lib_test/expect/test_str.ml

@@ -240,7 +240,12 @@ let%expect_test "match semantics" =
 
 let%expect_test "Group (or submatch)" =
   eq_match "\\(a\\)\\(a\\)?\\(b\\)" "ab";
-  [%expect {| |}]
+  [%expect {| |}];
+  eq_match "\\(foo" "foo";
+  [%expect {|
+    str: (Error "Failure(\"\\\\( group not closed by \\\\)\")")
+    re: (Error Re_private.Emacs.Parse_error)
+    |}]
 ;;
 
 let%expect_test "Character set" =