models: require belong_to associations on type_de_champ

- Make `type_de_champ.procedure` a requirement;
- Move the procedure_id assignation to `before_validation` (otherwise
the record is invalid, and never gets saved);

spec/factories/type_de_champ.rb

@@ -7,6 +7,8 @@ FactoryBot.define do
     mandatory { false }
     add_attribute(:private) { false }
 
+    association :procedure
+
     factory :type_de_champ_text do
       type_champ { TypeDeChamp.type_champs.fetch(:text) }
     end

spec/models/type_de_champ_shared_example.rb

@@ -156,14 +156,14 @@ shared_examples 'type_de_champ_spec' do
   describe "repetition" do
     let(:procedure) { create(:procedure) }
     let(:type_de_champ) { create(:type_de_champ_repetition, procedure: procedure) }
-    let(:type_de_champ_text) { create(:type_de_champ_text) }
+    let(:type_de_champ_text) { create(:type_de_champ_text, procedure: procedure) }
     let(:type_de_champ_integer_number_attrs) { attributes_for(:type_de_champ_integer_number) }
 
     it "associates nested types_de_champ to the parent procedure" do
       expect(type_de_champ.types_de_champ.size).to eq(0)
       expect(procedure.types_de_champ.size).to eq(1)
 
-      procedure.update(types_de_champ_attributes: [
+      procedure.update!(types_de_champ_attributes: [
         {
           id: type_de_champ.id,
           libelle: type_de_champ.libelle,