Change the default test locations to avoid confusion around the common
testing terms "unit" and "functional".
Add new rake tasks for the new locations, while maintaining backwards
compatibility with the old rake tasks.

New testing locations are as follows:

    app/models -> test/models (was test/units)
    app/helpers -> test/helpers (was test/units/helpers)
    app/controllers -> test/controllers (was test/functional)
    app/mailers -> test/mailers (was test/functional)