Use test-unit gem

test/unit.rb is minitest gem wrapper until Ruby 2.1. But test/unit.rb is
test-unit gem since Ruby 2.2. This change ensure using test-unit gem.

test-unit gem uses "module" to share tests not inheriting class.
Tests in parent classes aren't executed in test-unit gem. (They are
executed in minitest gem.)
Use auto test runner

It is for running before at_exit hook. Some libraries use at_exit hook
to finalize them. So it is safe to not use at_exit hook to running tests.
Merge pull request #22 from kou/use-test-unit-gem

I confirmed and will merge it.
Very very thanks for your contribution!!