Fix CI for TruffleRuby

[eregon]

Example failure:
https://github.com/ruby-syntax-tree/syntax_tree/actions/runs/4276174338/jobs/7444068581

$ bundle exec rake test
Coverage report generated for Unit Tests to /home/runner/work/syntax_tree/syntax_tree/coverage. 5963 / 14181 LOC (42.05%) covered.
Stopped processing SimpleCov as a previous error not related to SimpleCov has been detected
/home/runner/work/syntax_tree/syntax_tree/lib/syntax_tree/reflection.rb:179:in `<module:Reflection>': undefined method `statements' for nil:NilClass (NoMethodError)
	from /home/runner/work/syntax_tree/syntax_tree/lib/syntax_tree/reflection.rb:7:in `<module:SyntaxTree>'
	from /home/runner/work/syntax_tree/syntax_tree/lib/syntax_tree/reflection.rb:3:in `<top (required)>'
	from <internal:core> core/kernel.rb:234:in `gem_original_require'
	from /home/runner/work/syntax_tree/syntax_tree/test/test_helper.rb:17:in `<top (required)>'
	from <internal:core> core/kernel.rb:293:in `require_relative'
	from /home/runner/work/syntax_tree/syntax_tree/test/cli_test.rb:3:in `<top (required)>'
	from <internal:core> core/kernel.rb:234:in `gem_original_require'
	from /home/runner/work/syntax_tree/syntax_tree/vendor/bundle/truffleruby/3.1.3.3/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:21:in `block in <main>'
	from /home/runner/work/syntax_tree/syntax_tree/vendor/bundle/truffleruby/3.1.3.3/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in `select'
	from /home/runner/work/syntax_tree/syntax_tree/vendor/bundle/truffleruby/3.1.3.3/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in `<main>'
rake aborted!