Add a simple method to run regression tests

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: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5620)