Remove usage of assigns in controller specs (#30195)

spec/controllers/concerns/account_controller_concern_spec.rb

@@ -7,7 +7,7 @@ describe AccountControllerConcern do
     include AccountControllerConcern
 
     def success
-      head 200
+      render plain: @account.username # rubocop:disable RSpec/InstanceVariable
     end
   end
 
@@ -51,12 +51,13 @@ describe AccountControllerConcern do
   context 'when account is not suspended' do
     let(:account) { Fabricate(:account, username: 'username') }
 
-    it 'assigns @account, returns success, and sets link headers' do
+    it 'Prepares the account, returns success, and sets link headers' do
       get 'success', params: { account_username: account.username }
 
-      expect(assigns(:account)).to eq account
       expect(response).to have_http_status(200)
       expect(response.headers['Link'].to_s).to eq(expected_link_headers)
+      expect(response.body)
+        .to include(account.username)
     end
 
     def expected_link_headers