WFE2 unit test cleanup (part 2) (#2977)
WFE2 unit test cleanup (part 2) (#2977)

This is the second half of a clean-up of the WFE2 unit tests that were copied over from the original WFE implementation.

This PR covers TestChallenge, TestAuthorization, and TestGetCertificate.

Resolves #2928
jsha/boulder,jcjones/boulder,jcjones/boulder,jfrazelle/boulder,letsencrypt/boulder,jcjones/boulder,ibukanov/boulder,letsencrypt/boulder,bmw/boulder,letsencrypt/boulder,jfrazelle/boulder,lucasallan/boulder,jcjones/boulder,jfrazelle/boulder,jsha/boulder,bmw/boulder,lucasallan/boulder,lucasallan/boulder,jfrazelle/boulder,ibukanov/boulder,jsha/boulder,ibukanov/boulder,bmw/boulder