Enable memory debugging while testing

Pre 1.1.0, 'make test' would set the environment variable
OPENSSL_DEBUG_MEMORY to "on".  This got lost when translating the old
build files to the new templates.  This changes reintroduces that
variable.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1840)