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>
diff --git a/programs/ssl/ssl_test_lib.c b/programs/ssl/ssl_test_lib.c
index e3645f3..897afc6 100644
--- a/programs/ssl/ssl_test_lib.c
+++ b/programs/ssl/ssl_test_lib.c
@@ -1,5 +1,8 @@
 /*
- *  Common code for SSL test programs
+ *  Common code library for SSL test programs.
+ *
+ *  In addition to the functions in this file, there is shared source code
+ *  that cannot be compiled separately in "ssl_test_common_source.c".
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0