AbstractSecurityTest#getUserManager(Root) doesn't create new instances

i just happened to struggle with my own test-setup that remembers the user manager created for the specified Root in AbstractSecurityTest.getUserManager. When called with a different root later on it will return an instance associated with a wrong root instance.

the fix could be any of the following:

• don't remember the user manager
• only remember instance for the root field and make sure only user mangers for the right root instance are recycled.
• drop the Root param altogether (or make it optional)