tests: switch to minitest

Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5,
the shim has broken some compatibility with Test::Unit::TestCase in some
scenarios.

Adjusts the test suite to support Minitest 5's syntax.
remove dependency on resque/server/test_helper

The Resque developers have removed resque/server/test_helper.rb in
Resque's master branch when they split out resque-web (resque commit
b2ca4ceb9457242fc0170d95b3bce9d427baa387).

It seems that this test_helper is an internal implementation detail and
that external consumers should not rely on it.

Additionally, in Resque v1.25.2, resque/server/test_helper.rb adjusts
the Test::Unit::TestCase class. Now that resque-cleaner uses Minitest,
we want to adjust the Minitest::Spec class instead.