Add JSON parser failure tests for whitespace consumption

c++/src/capnp/compat/json-test.c++

@@ -294,6 +294,15 @@ KJ_TEST("basic json decoding") {
     KJ_EXPECT(root.getObject().size() == 0);
   }
 
+  {
+    MallocMessageBuilder message;
+    auto root = message.initRoot<JsonValue>();
+
+    json.decodeRaw("\r\n\t {\r\n\t }\r\n\t ", root);
+    KJ_EXPECT(root.which() == JsonValue::OBJECT, (uint)root.which());
+    KJ_EXPECT(root.getObject().size() == 0);
+  }
+
   {
     MallocMessageBuilder message;
     auto root = message.initRoot<JsonValue>();
@@ -428,6 +437,8 @@ KJ_TEST("basic json decoding") {
     KJ_EXPECT_THROW_MESSAGE("Invalid hex", json.decodeRaw(R"("\u12zz")", root));
     KJ_EXPECT_THROW_MESSAGE("ends prematurely", json.decodeRaw("-", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("--", root));
+    KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("\f{}", root));
+    KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("{\v}", root));
   }
 }