Instance vars no longer emit warnings in Ruby 3

From: ruby/ruby@01b7d5a

Author: benoittgt

spec/rspec/support/spec/stderr_splitter_spec.rb

@@ -88,7 +88,7 @@
 
   unless defined?(RUBY_ENGINE) && RUBY_ENGINE == 'rbx'
     it 'will fail an example which generates a warning' do
-      true unless @undefined
+      true unless $undefined
       expect { splitter.verify_no_warnings! }.to raise_error(/Warnings were generated:/)
     end
   end