DEV: Add system spec to check core features are working fine (#40)

Author: Flink

spec/system/core_features_spec.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+RSpec.describe "Core features", type: :system do
+  before { upload_theme_or_component }
+
+  it_behaves_like "having working core features"
+end