models: require belong_to associations on champ

- Make `champ.dossier` a requirement;
- Move the dossier_id assignation to `before_validation` (otherwise
the record is invalid, and never gets saved);
- Allow specs to only build the champ (instead of saving it to the
database), which bypasses the requirement to have a dossier.

spec/jobs/virus_scanner_job_spec.rb

@@ -2,7 +2,7 @@ RSpec.describe VirusScannerJob, type: :job do
   include ActiveJob::TestHelper
 
   let(:champ) do
-    champ = create(:champ, :piece_justificative)
+    champ = create(:champ_piece_justificative)
     champ.piece_justificative_file.attach(io: StringIO.new("toto"), filename: "toto.txt", content_type: "text/plain")
     champ.save
     champ