Add unity assertion failure callback handler.

frameworks\utest/source/unity_handler.cpp

@@ -0,0 +1,25 @@
+/****************************************************************************
+ * Copyright (c) 2015, ARM Limited, All Rights Reserved
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ ****************************************************************************
+ */
+
+ #include "mbed-test-async/async_test.h"
+
+extern "C"
+void mbed_test_unity_assert_failure()
+{
+    mbed::test::v0::TestHarness::raise_failure(mbed::test::v0::FAILURE_ASSERTION);
+}