factories: cleanup Administrateur, Instructeur and Expert factories

This changes:

- avoids relying on application code to create mock objects,
- allows to build Administrateur or Instructeurs without saving them.

spec/models/administrateur_spec.rb

@@ -1,7 +1,7 @@
 describe Administrateur, type: :model do
   let(:administration) { create(:administration) }
 
-  describe 'assocations' do
+  describe 'associations' do
     it { is_expected.to have_and_belong_to_many(:instructeurs) }
     it { is_expected.to have_many(:procedures) }
   end