CI: Fix CI failures with minitest v6

[taketo1113]

This Pull Request fixes CI failures caused by minitest v6.

With minitest v6, the following errors occur in CI:

  1) Error:
Slim::Generators::AuthenticationGeneratorTest#test_should_generate_SlimLint_valid_templates:
NoMethodError: undefined method 'stub' for an instance of Rails::Generators::AuthenticationGenerator
    test/lib/generators/slim/authentication_generator_test.rb:39:in 'Slim::Generators::AuthenticationGeneratorTest#run_generator_instance'
    test/lib/generators/slim/authentication_generator_test.rb:26:in 'block in <class:AuthenticationGeneratorTest>'

  2) Error:
Slim::Generators::AuthenticationGeneratorTest#test_should_invoke_template_engine:
NoMethodError: undefined method 'stub' for an instance of Rails::Generators::AuthenticationGenerator
    test/lib/generators/slim/authentication_generator_test.rb:39:in 'Slim::Generators::AuthenticationGeneratorTest#run_generator_instance'
    test/lib/generators/slim/authentication_generator_test.rb:18:in 'block in <class:AuthenticationGeneratorTest>'