Add minitest to development dependency
Add minitest to development dependency

Now, minitest is gemified (not included in ruby-core)

```
$ bundle exec rake test
/Users/sue445/.rbenv/versions/3.0.3/bin/ruby -w -I"lib" /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb "spec/connpass_spec.rb"
/Users/sue445/workspace/github.com/deeeki/connpass/spec/spec_helper.rb:12:in `require': cannot load such file -- minitest/autorun (LoadError)
	from /Users/sue445/workspace/github.com/deeeki/connpass/spec/spec_helper.rb:12:in `<top (required)>'
	from /Users/sue445/workspace/github.com/deeeki/connpass/spec/connpass_spec.rb:2:in `require'
	from /Users/sue445/workspace/github.com/deeeki/connpass/spec/connpass_spec.rb:2:in `<top (required)>'
	from /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:21:in `require'
	from /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:21:in `block in <main>'
	from /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in `select'
	from /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in `<main>'
rake aborted!
Command failed with status (1): [ruby -w -I"lib" /Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb "spec/connpass_spec.rb" ]
/Users/sue445/workspace/github.com/deeeki/connpass/vendor/bundle/ruby/3.0.0/gems/rake-13.0.6/exe/rake:27:in `<top (required)>'
/Users/sue445/.rbenv/versions/3.0.3/bin/bundle:23:in `load'
/Users/sue445/.rbenv/versions/3.0.3/bin/bundle:23:in `<main>'
Tasks: TOP => test
(See full trace by running task with --trace)
```

deeeki/connpass