Refactor and improve tests (#17386)

* 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.

spec/policies/report_policy_spec.rb

@@ -6,7 +6,7 @@ require 'pundit/rspec'
 RSpec.describe ReportPolicy do
   let(:subject) { described_class }
   let(:admin)   { Fabricate(:user, admin: true).account }
-  let(:john)    { Fabricate(:user).account }
+  let(:john)    { Fabricate(:account) }
 
   permissions :update?, :index?, :show? do
     context 'staff?' do