Disable FF when generating fixtures

Since a lot of tests depend on certain DOM elements to be presented in
the markup for the snippet view, the fixtures should be generated with
FF disabled to fallback to the HAML markup before the Vue components
get implemented

spec/frontend/fixtures/snippet.rb

@@ -17,6 +17,7 @@ describe SnippetsController, '(JavaScript fixtures)', type: :controller do
   end
 
   before do
+    stub_feature_flags(snippets_vue: false)
     sign_in(admin)
     allow(Discussion).to receive(:build_discussion_id).and_return(['discussionid:ceterumcenseo'])
   end