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/timelines/home_controller_spec.rb

@@ -5,7 +5,7 @@ require 'rails_helper'
 describe Api::V1::Timelines::HomeController do
   render_views
 
-  let(:user) { Fabricate(:user, account: Fabricate(:account, username: 'alice'), current_sign_in_at: 1.day.ago) }
+  let(:user) { Fabricate(:user, current_sign_in_at: 1.day.ago) }
 
   before do
     allow(controller).to receive(:doorkeeper_token) { token }

spec/controllers/api/v1/timelines/list_controller_spec.rb

@@ -5,7 +5,7 @@ require 'rails_helper'
 describe Api::V1::Timelines::ListController do
   render_views
 
-  let(:user) { Fabricate(:user, account: Fabricate(:account, username: 'alice')) }
+  let(:user) { Fabricate(:user) }
   let(:list) { Fabricate(:list, account: user.account) }
 
   before do
@@ -30,7 +30,7 @@ describe Api::V1::Timelines::ListController do
   end
 
   context 'with the wrong user context' do
-    let(:other_user) { Fabricate(:user, account: Fabricate(:account, username: 'bob')) }
+    let(:other_user) { Fabricate(:user) }
     let(:token)      { Fabricate(:accessible_access_token, resource_owner_id: other_user.id, scopes: 'read') }
 
     describe 'GET #show' do

spec/controllers/api/v1/timelines/public_controller_spec.rb

@@ -5,7 +5,7 @@ require 'rails_helper'
 describe Api::V1::Timelines::PublicController do
   render_views
 
-  let(:user) { Fabricate(:user, account: Fabricate(:account, username: 'alice')) }
+  let(:user) { Fabricate(:user) }
 
   before do
     allow(controller).to receive(:doorkeeper_token) { token }

spec/controllers/api/v1/timelines/tag_controller_spec.rb

@@ -5,7 +5,7 @@ require 'rails_helper'
 describe Api::V1::Timelines::TagController do
   render_views
 
-  let(:user) { Fabricate(:user, account: Fabricate(:account, username: 'alice')) }
+  let(:user) { Fabricate(:user) }
 
   before do
     allow(controller).to receive(:doorkeeper_token) { token }