Add tests for DigestMechanismFactory

Add tests for DigestMechanismFactory. For inspiration take a look at BasicMechanismFactoryContractTest.

Also add a test case to both which tests that if different challenge prefix gets used (so eg. try to use BasicMechanismFactory to create authentication mechanism with name "DIGEST" and vice versa), the factory returns null. But with appropriate prefix the factory will create digest / basic authentication mechanism as expected.