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: #7194 (closed) CI: https://ci.nodejs.org/job/node-test-pull-request/2948/