Suppress `parser` warnings.

Library/Homebrew/test/rubocop_spec.rb

@@ -14,7 +14,7 @@ describe "RuboCop" do
     end
 
     it "loads all Formula cops without errors" do
-      stdout, stderr, status = Open3.capture3("rubocop", TEST_FIXTURE_DIR/"testball.rb")
+      stdout, stderr, status = Open3.capture3(RUBY_PATH, "-W0", "-S", "rubocop", TEST_FIXTURE_DIR/"testball.rb")
       expect(stderr).to be_empty
       expect(stdout).to include("no offenses detected")
       expect(status).to be_a_success