This is only useful when building shared libraries.  This allows us to
run our tests against newer libraries when the time comes.  Simply do
this:

    OPENSSL_REGRESSION=/other/OpenSSL/build/tree make test

($OPENSSL_REGRESSION *must* be an absolute path)
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5619)