Add and apply RSpec/FilePath rubocop rule

This way we make sure editors which support navigating between one class
and its test file can find the alternative files.

spec/controllers/sdg/goals_controller_spec.rb

@@ -0,0 +1,13 @@
+require "rails_helper"
+
+describe SDG::GoalsController do
+  context "featured disabled" do
+    before do
+      Setting["feature.sdg"] = false
+    end
+
+    it "raises feature disabled" do
+      expect { get :index }.to raise_exception(FeatureFlags::FeatureDisabled)
+    end
+  end
+end