Some fixes for Test::Unit/Shoulda/RR test cases

These problems have been around for sometime:

1. Include `RR::Adapters::TestUnit` instead of `RR::Adapters::RRMethods` in `test_config.rb` 

`RR::Adapters::TestUnit` verifies and resets expectations (it includes `RR::Adapters::RRMethods`) and defines `assert_received`. 

2. Don't `require shoulda/active_record` in `test_config.rb`

I don't think this has been necessary since shoulda was separated into context and matchers

3. `require test/unit` in `test_config.rb` 

I looked into this a while ago and, if I recall, this is necessary now that `MiniTest` is part of ruby (as it now provides `Test::Unit` -this could have implications with the test unit generator, but I'm not sure). Without it, no tests will be run.