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/services/unfollow_service_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe UnfollowService, type: :service do
   subject { UnfollowService.new }
 
   describe 'local' do
-    let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob')).account }
+    let(:bob) { Fabricate(:account, username: 'bob') }
 
     before do
       sender.follow!(bob)
@@ -19,7 +19,7 @@ RSpec.describe UnfollowService, type: :service do
   end
 
   describe 'remote ActivityPub' do
-    let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox')).account }
+    let(:bob) { Fabricate(:account, username: 'bob', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox') }
 
     before do
       sender.follow!(bob)
@@ -37,7 +37,7 @@ RSpec.describe UnfollowService, type: :service do
   end
 
   describe 'remote ActivityPub (reverse)' do
-    let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox')).account }
+    let(:bob) { Fabricate(:account, username: 'bob', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox') }
 
     before do
       bob.follow!(sender)