tests: Add helpers.c and helpers.h files

The purpose of helpers.c file is to contain the helper
functions that have been in helpers.function so far and
that are not related to the mechanism of unit test
execution and not related to random number generation
(will be moved in a dedicated file).

The purpose of helpers.h is to contain the interface
exposed by helpers.c thus helper function prototypes.

Make the changes in the build systems (make and cmake)
to build helpers.c and link it to test executables
along with mbedtls library.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

tests/src/helpers.c

@@ -0,0 +1,19 @@
+/*  Copyright (C) 2020, 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.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#include <test/helpers.h>