Add conditional unit testing interface.

Don't call internal functions directly call them through
SSL_test_functions(). This also makes unit testing work on
Windows and platforms that don't export internal functions
from shared libraries.

By default unit testing is not enabled: it requires the compile
time option "enable-unit-test".
Reviewed-by: Geoff Thorpe <geoff@openssl.org>
(cherry picked from commit e0fc7961c4)

Conflicts:

	ssl/Makefile
	util/mkdef.pl

ssl/ssl.h

@@ -2056,6 +2056,10 @@ int SSL_set_session_secret_cb(SSL *s, tls_session_secret_cb_fn tls_session_secre
 void SSL_set_debug(SSL *s, int debug);
 int SSL_cache_hit(SSL *s);
 
+#ifndef OPENSSL_NO_UNIT_TEST
+const struct openssl_ssl_test_functions *SSL_test_functions(void);
+#endif
+
 /* BEGIN ERROR CODES */
 /* The following lines are auto generated by the script mkerr.pl. Any changes
  * made after this point may be overwritten when the script is next run.