Don't call setenv in gost2814789t.c

The call to setenv in gost2814789t.c is not portable and may
not reflect the location of the GOST ENGINE on all platforms anyway.
Instead set OPENSSL_ENGINES in test/Makefile
Reviewed-by: Geoff Thorpe <geoff@openssl.org>
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>