Add unit tests for the AuthMgrBE.

This CL adds:
1. test-util crate under authmgr-be, which tests the AuthMgr BE, given
   and instance of AuthMgrBE and example pvms/clients.
2. test crate under authmgr-be-impl which invokes those tests by
   providing an example instance of AuthMgrBE and example instances of
   pvms/clients

Test: will be added in the upcoming CLs
Bug: 369145328
Change-Id: I3c318cca34933da64c05d4d7261e195564ccaf5f
5 files changed