DEV: Skip core features specs on stable (#269)

The shared example is not available in stable yet

Author: tgxworld

spec/system/core_features_spec.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe "Core features", type: :system do
+  # TODO: Stop skipping when shared example is available in stable
+  next if Discourse.git_branch == "stable"
+
   before { enable_current_plugin }
 
   it_behaves_like "having working core features"