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 e0fc7961)

Conflicts:

	ssl/heartbeat_test.c
	ssl/ssl.h
	util/mkdef.pl