* Change account and user fabricators to simplify and improve tests
- `Fabricate(:account)` implicitly fabricates an associated `user` if
no `domain` attribute is given (an account with `domain: nil` is
considered a local account, but no user record was created), unless
`user: nil` is passed
- `Fabricate(:account, user: Fabricate(:user))` should still be possible
but is discouraged.
* Fix and refactor tests
- avoid passing unneeded attributes to `Fabricate(:user)` or
`Fabricate(:account)`
- avoid embedding `Fabricate(:user)` into a `Fabricate(:account)` or the other
way around
- prefer `Fabricate(:user, account_attributes: …)` to
`Fabricate(:user, account: Fabricate(:account, …)`
- also, some tests were using remote accounts with local user records, which is
not representative of production code.
