Include test directory in load path

Because Rails-generated tests start with `require 'test_helper'`, we need to include the test directory in the load path.

Without this patch, auto-generated tests fail with:
```
test/unit/user_test.rb:1:in `require': cannot load such file -- test_helper (LoadError)
	from test/unit/user_test.rb:1:in `<main>'
```

The alternative is to ask every user to change to `require_relative`, and I assert that users should not have to change your tests for them to work with Guard.