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/fabricators/account_fabricator.rb

@@ -10,4 +10,5 @@ Fabricator(:account) do
   private_key         { private_key }
   suspended_at        { |attrs| attrs[:suspended] ? Time.now.utc : nil }
   silenced_at         { |attrs| attrs[:silenced] ? Time.now.utc : nil }
+  user                { |attrs| attrs[:domain].nil? ? Fabricate.build(:user, account: nil) : nil }
 end

spec/fabricators/user_fabricator.rb

@@ -1,5 +1,5 @@
 Fabricator(:user) do
-  account
+  account      { Fabricate.build(:account, user: nil) }
   email        { sequence(:email) { |i| "#{i}#{Faker::Internet.email}" } }
   password     "123456789"
   confirmed_at { Time.zone.now }