Re-enabled UT that was disabled and fixed it.

JD disabled this UT
https://github.com/sonatype/nexus-oss/commit/46662af44f869a17f0c30becb61ad55949ecaedb

But it turns out that everything was here actually,
no baggage needed from test-jar, but only some
annotations were missing on _already copy pasted_
mock user manager.