16470: Fixes test on user model.

16470: Fixes test on user model.

Rails 5.1 deprecated the attr_changed? in favor of more explicit methods
because there was ambiguity when called from an 'after' or 'before' callback.
The test UsersTest#test_cannot_set_is_active_to_false_directly started failing
because User.setup is called from both types of callbacks, so its internal
checks weren't passing in some cases.

Also, avoids doing unnecessary queries to get the 'All users' group.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>