Improve test coverage with sling.auth.core

according to https://sonarcloud.io/dashboard?id=apache_sling-org-apache-sling-auth-core the current code coverage is just 29%. for a bundle as crucial as the sling authentication code base, this should be heavily increased and should in particular provide full coverage for those parts that are critical for the overall stability and performance. afaik there pieces in the SlingAuthenticator that look wired but cannot be touched without introducing major regressions.