Adopted typical Ruby conventions for test running.

This commit restructures the test case files and
removes test-rails.rb in favor of running tests
via Rake, with helper and initialization code
defined in the test_helper.rb` file.

It also…

* renames all test case filenames to end with
  "_test.rb", to aid in filesystem globbing;

* moves test case files into either a "rails" or
  "application" directory, based on the scope of
  the test;

* no longer requires you to include new test cases
  into a parent TestCase instance. Simply create a
  MiniTest::Unit::TestCase instance for each test
  case.