Add test for RuboCop `$LOAD_PATH`.

Library/Homebrew/test/rubocop_spec.rb

+require "open3"
+
+describe "RuboCop" do
+  context "when calling `rubocop` outside of the Homebrew environment" do
+    before do
+      ENV.keys.each do |key|
+        ENV.delete(key) if key.start_with?("HOMEBREW_")
+      end
+
+      ENV["XDG_CACHE_HOME"] = "#{HOMEBREW_CACHE}/style"
+    end
+
+    it "loads all Formula cops without errors" do
+      _, _, status = Open3.capture3("rubocop", TEST_FIXTURE_DIR/"testball.rb")
+      expect(status).to be_a_success
+    end
+  end
+end