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/controllers/api/v1/conversations_controller_spec.rb

@@ -3,9 +3,9 @@ require 'rails_helper'
 RSpec.describe Api::V1::ConversationsController, type: :controller do
   render_views
 
-  let!(:user) { Fabricate(:user, account: Fabricate(:account, username: 'alice')) }
+  let!(:user) { Fabricate(:user, account_attributes: { username: 'alice' }) }
   let(:token) { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
-  let(:other) { Fabricate(:user, account: Fabricate(:account, username: 'bob')) }
+  let(:other) { Fabricate(:user) }
 
   before do
     allow(controller).to receive(:doorkeeper_token) { token }