There's no reason for guides to reference old behaviors. They should
be current as of the versions of Rails that they ship with, and including
older information just clutters thing.
I discussed this change with @fxn and he agrees.
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)
