test: fix test failure due to expired certificates

Back-port commit 76f40f77 ("test: stronger crypto in test fixtures") from
the master branch.

Pushes back the expiration date of test/fixtures/keys/ca2-crl.pem to
2018, fixing a CRL_HAS_EXPIRED error in simple/test-tls-server-verify.

Fixes: https://github.com/nodejs/node/issues/7194
PR-URL: https://github.com/nodejs/node/pull/7195


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>