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/heartbeat_test.c

@@ -38,14 +38,17 @@
  * http://mike-bland.com/tags/heartbleed.html
  */
 
+#define OPENSSL_UNIT_TEST
+
 #include "../test/testutil.h"
+
 #include "../ssl/ssl_locl.h"
 #include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#if !defined(OPENSSL_NO_HEARTBEATS) && !defined(OPENSSL_SYS_WINDOWS)
+#if !defined(OPENSSL_NO_HEARTBEATS) && !defined(OPENSSL_NO_UNIT_TEST)
 
 /* As per https://tools.ietf.org/html/rfc6520#section-4 */
 #define MIN_PADDING_SIZE	16