Whenever possible, reuse User objects in unit tests

Whenever possible, reuse User objects in unit tests

The unit tests spend nearly half of their run time resetting the user table for
each test. But the majority of tests do not depend on the user table having the
exact value that the setup code resets it to, and do not need to modify the
user objects they require to run.

Fix that by providing an API for tests to get User objects, and to indicate
whether the User object will be subject to destructive modification or not.
This allows User objects to be reused across multiple unit tests.

Change-Id: I17ef1f519759c5e7796c259282afe730ef722e96