Updated CRAMMD5Authentication tests to additionally run against test authenticator module.

Review: https://reviews.apache.org/r/27619
Updated CRAMMD5Authentication tests to additionally run against test authenticatee module.

CRAMMD5Authentication tests now cover all default and modularized
authenticator/authenticatee combinations.
Also adds a test for missing credential for the authenticatee.

Review: https://reviews.apache.org/r/27651