Add support for test-unit test runner

[ArturT]

test-unit test runner:

• https://github.com/test-unit/test-unit
• https://rubygems.org/gems/test-unit/versions

Docs: http://www.rubydoc.info/github/test-unit/test-unit/Test/Unit/TestCase

Here is a call order, some of them should be useful:

startup
setup
test_my_method1
cleanup
teardown
setup
test_my_method2
cleanup
teardown
shutdown

Important things to support:

• shared examples Shared examples (like rspec) test-unit/test-unit#68
• https://github.com/michaelgpearce/shared_should