Add a file to the transcode fixture

I was seeing a test failure in CI

   No route matches [GET] "/rails/active_storage/disk.../track.mp3

I'm hoping that explicitly generating the file rather than relying on
TranscodeJob might fix it.

test/factories/transcode.rb

@@ -4,5 +4,13 @@
   factory :transcode do
     track
     format { 0 }
+
+    after(:build) do |transcode|
+      transcode.file.attach(
+        io: Rails.root.join('test/fixtures/files/track.mp3').open,
+        filename: 'track.mp3',
+        content_type: 'audio/mpeg'
+      )
+    end
   end
 end

test/system/buying_an_album_test.rb

@@ -6,7 +6,7 @@ class BuyingAnAlbumTest < ApplicationSystemTestCase
   setup do
     sign_in_as(create(:user))
     @album = create(:album_with_tracks)
-    TranscodeJob.perform_now(@album.tracks.first)
+    @album.tracks.first.transcodes << create(:transcode)
   end
 
   test 'viewing the album' do