Add OCSP stapling unit tests

[kenjenkins]

I thought it might be nice to add a few more unit tests for ocsp.go. (This has been at the back of my mind since #245, and I was curious to learn a little bit more about how OCSP works.) Please let me know if you have any concerns, either on the high-level approach or on specific details.

These test cases exercise some more of the logic starting from the stapleOCSP() method, using a few more test certificates and an httptest.Server acting as the OCSP responder.

These tests are very simplistic:

• a "good" OCSP response should be stapled
• a "revoked" OCSP response should not be stapled
• the DisableStapling option should be honored
• OCSP stapling requires an issuing certificate

I haven't made any attempt to provide sensible timestamps, either in the test certificates or in the OCSP responses, as it's always a little tricky writing unit tests where time.Now() is involved.

I believe this should bring unit test coverage for ocsp.go from 21% to 70%.

[kenjenkins]

Hi @mholt, please let me know if you have any feedback about this idea. But feel free to close if you think it's not worth bothering with something like this.

[mholt]

Hey @kenjenkins ! So sorry. I am actually very interested in this. We just had a baby a couple months early and she's about to come home finally, so I've been very busy accumulating a backlog of issues and PRs to review.

I've actually been wanting to review this first, since it looks solid and I love tests. I'll do that in a moment. :)

[kenjenkins]

Thanks @mholt, and no need to apologize. Congratulations on your baby girl!