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/concerns/account_controller_concern_spec.rb

@@ -17,7 +17,7 @@ describe ApplicationController, type: :controller do
 
   context 'when account is suspended' do
     it 'returns http gone' do
-      account = Fabricate(:account, suspended: true, user: Fabricate(:user))
+      account = Fabricate(:account, suspended: true)
       get 'success', params: { account_username: account.username }
       expect(response).to have_http_status(410)
     end
@@ -33,19 +33,19 @@ describe ApplicationController, type: :controller do
 
   context 'when account is not suspended' do
     it 'assigns @account' do
-      account = Fabricate(:account, user: Fabricate(:user))
+      account = Fabricate(:account)
       get 'success', params: { account_username: account.username }
       expect(assigns(:account)).to eq account
     end
 
     it 'sets link headers' do
-      account = Fabricate(:account, username: 'username', user: Fabricate(:user))
+      account = Fabricate(:account, username: 'username')
       get 'success', params: { account_username: 'username' }
       expect(response.headers['Link'].to_s).to eq '<http://test.host/.well-known/webfinger?resource=acct%3Ausername%40cb6e6126.ngrok.io>; rel="lrdd"; type="application/jrd+json", <https://cb6e6126.ngrok.io/users/username>; rel="alternate"; type="application/activity+json"'
     end
 
     it 'returns http success' do
-      account = Fabricate(:account, user: Fabricate(:user))
+      account = Fabricate(:account)
       get 'success', params: { account_username: account.username }
       expect(response).to have_http_status(200)
     end