DEV: Fix flaky signed_url_for_path spec

AWS gem uses internally `Time.now` to generate the presigned URLs, so often two consecutive calls with the same params would give different results.

spec/requests/uploads_controller_spec.rb

@@ -386,6 +386,7 @@ describe UploadsController do
         end
 
         it "redirects to the signed_url_for_path" do
+          freeze_time
           get upload.short_path
 
           expect(response).to redirect_to(Discourse.store.signed_url_for_path(Discourse.store.get_path_for_upload(upload)))