libcrypto: Install tests in the tests package

libcrypto's Makefile.inc used PACKAGE=openssl, which overrides the
PACKAGE=tests in libcrypto/tests/Makefile. Use PACKAGE?=openssl
instead to avoid this. This puts the OpenSSL tests in the tests
package where they belong.

MFC after: 1 day
Reviewed by: manu, ngie
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53595