Fix AuthenticatedTestHelper inclusion in test_helper.b

The test couldn't be run since the AuthenticatedTestHelper module
could not be found.
Fixed fixtures that were failing already existing tests
Added Backtrace filtering to Test::Unit tests

The Test::Unit tests use MiniTest.filter_backtrace method to filter
backtraces. The MinitestBacktraceFilter module included in the commit
wraps that method inside another method which adds
Rails.backtrace_cleaner filtering.
Added shoulda gem as a dependency
Added factory_girl gem as a dependency
Initial (Skill) factory for factory_girl