Add test for list project users when no user

There was no test that shows what happens when you list project users
(GET /v2.0/tenants/{tenant_id}/users) and a user with a role on the
project doesn't exist. The server returns a 404 Not Found for the user.

bp no-check-id

Change-Id: Iba8836200c22eb5d744b42114cc8c6e864c3e02f
Merge "Add test for list project users when no user"