add spec for archive weight estimation

spec/helpers/archive_helper_spec.rb

@@ -0,0 +1,24 @@
+describe ArchiveHelper, type: :helper do
+  describe ".estimate_weight" do
+    let(:nb_dossiers_termines) { 5 }
+    let(:average_dossier_weight) { 2 }
+
+    context 'when archive exist and available' do
+      let(:archive) { build(:archive, :generated) }
+      before do
+        allow_any_instance_of(Archive).to receive(:available?).and_return(true)
+      end
+
+      it 'returns real archive weight' do
+        expect(estimate_weight(archive, nb_dossiers_termines, average_dossier_weight)).to eq nil
+      end
+    end
+
+    context 'when archive has not been created' do
+      let(:archive) { nil }
+      it 'returns estimation' do
+        expect(estimate_weight(archive, nb_dossiers_termines, average_dossier_weight)).to eq 10
+      end
+    end
+  end
+end