Create ssl_helpers.c to hold functions of TLS connection

test_suite_ssl.function contains many functions that are used to set
up a TLS connection. To reduce its file size, those functions would
be moved to ssl_helpers.c under tests/src. As the start of this
implementation, some necessary header files are moved in advance.

Signed-off-by: Yanray Wang <yanray.wang@arm.com>

tests/include/test/ssl_helpers.h

@@ -0,0 +1,28 @@
+/** \file ssl_helpers.h
+ *
+ * \brief This file contains helper functions to set up a TLS connection.
+ */
+
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+
+#ifndef SSL_HELPERS_H
+#define SSL_HELPERS_H
+
+#include <test/helpers.h>
+
+#endif /* SSL_HELPERS_H */