Instance vars no longer emit warnings in Ruby 3

Related: ruby/ruby@01b7d5a

Author: benoittgt

features/command_line/warnings_option.feature

@@ -8,7 +8,7 @@ Feature: `--warnings` option (run with warnings enabled)
       """ruby
       RSpec.describe do
         it 'generates warning' do
-          @undefined
+          $undefined
         end
       end
       """
@@ -21,7 +21,7 @@ Feature: `--warnings` option (run with warnings enabled)
       """ruby
       RSpec.describe do
         it 'generates warning' do
-          @undefined
+          $undefined
         end
       end
       """