ssl_test_common_source.c: new source file for common SSL test code

Create a new source file for code that is shared between ssl_client2.c
and ssl_server2.c, but cannot be compiled separately and instead needs
to be #include'd in each program that uses it.

This is for code that references macros or types that are defined
differently in the two programs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

programs/ssl/ssl_test_common_source.c

@@ -0,0 +1,25 @@
+/*
+ *  Common source code for SSL test programs. This file is included by
+ *  both ssl_client2.c and ssl_server2.c and is intended for source
+ *  code that is textually identical in both programs, but that cannot be
+ *  compiled separately because it refers to types or macros that are
+ *  different in the two programs, or because it would have an incomplete
+ *  type.
+ *
+ *  This file is meant to be #include'd and cannot be compiled separately.
+ *
+ *  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.
+ */