Simplify test for CRL signer. (#4987)
Simplify test for CRL signer. (#4987)

This mocks out the signer type rather than mocking out the pkcs11
object, making the test less dependent on the internals of our
pkcs11helpers package.

Part of refactorings related to #4918.
jcjones/boulder,bmw/boulder,letsencrypt/boulder,bmw/boulder,jsha/boulder,jcjones/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,bmw/boulder,jcjones/boulder,letsencrypt/boulder