ssl_test_lib.[hc]: 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. This commit only creates the file; subsequent
commits will populate it.

Add it to the official build systems (make, cmake, VS).

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

programs/ssl/ssl_test_lib.h

@@ -0,0 +1,23 @@
+/*
+ *  Common code for SSL test programs
+ *
+ *  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 MBEDTLS_PROGRAMS_SSL_SSL_TEST_LIB_H
+#define MBEDTLS_PROGRAMS_SSL_SSL_TEST_LIB_H
+
+#endif /* MBEDTLS_PROGRAMS_SSL_SSL_TEST_LIB_H */