`require "webrick/utils.rb"` should load `lib/webrick/utils.rb`, but on
some CI machines, it wrongly loads `test/webrick/utils.rb`, which caused
circular require warning:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20190825T093007Z.log.html.gz
```
/data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/rubygems/core_ext/kernel_require.rb:61: warning: /data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/rubygems/core_ext/kernel_require.rb:61: warning: loading in progress, circular require considered harmful - /data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/webrick.rb
```