features: refactor the editor specs

- Add an `add_champ` helper
- Stop relying on a default champ being created
- Wait for all champs to be created by watching the "Add champ" button 

This should fix the flakiness of these tests.

Refs #4417

spec/features/admin/procedure_creation_spec.rb

@@ -69,14 +69,12 @@ feature 'As an administrateur I wanna create a new procedure', js: true do
       page.refresh
       expect(page).to have_current_path(champs_procedure_path(Procedure.last))
 
-      expect(page).to have_selector('#champ-0-libelle')
+      add_champ(remove_flash_message: true)
       fill_in 'champ-0-libelle', with: 'libelle de champ'
       blur
       expect(page).to have_content('Formulaire enregistré')
 
-      within '.buttons' do
-        click_on 'Ajouter un champ'
-      end
+      add_champ
       expect(page).to have_selector('#champ-1-libelle')
 
       click_on Procedure.last.libelle
@@ -91,6 +89,7 @@ feature 'As an administrateur I wanna create a new procedure', js: true do
     scenario 'After adding champ and file, make publication' do
       page.refresh
 
+      add_champ(remove_flash_message: true)
       fill_in 'champ-0-libelle', with: 'libelle de champ'
       blur
       expect(page).to have_content('Formulaire enregistré')