This introduces three new test helper methods:

TestCase.create_certificate()
TestCase.create_certificate_bundle()
TestCase.create_certificate_fingerprints()

These are used to create certificate-related objects for testing, with

pre-populated data.
create_certificate() defaults to hard-coded certificate values but no

certificate data. If cert_data is passed in, then default values are

loaded from the data instead.

Unit tests pass.
Used some of these in in-progress code.