Extract test framework detection code

Add tests which ensure that shoulda-context knows how to detect which
test framework is loaded currently, under different environments. These
environments are supported:

* ActiveSupport 3.0 (Test::Unit 2.x)
* ActiveSupport 3.1 (Test::Unit 2.x)
* ActiveSupport 3.2 (Test::Unit 2.x)
* ActiveSupport 4.0 (MiniTest 4.x)
Integrate with Rails 4.1

Rails 4.1 requires Minitest 5, which changes MiniTest::Unit::TestCase to
Minitest::Test. Hence we have to inject shoulda-context into it
specifically.