Remove rack/test from rspec config

We are not using it (and may never will). Remove it.
Add a 'all' unit tests rake task.

So we can run all unit tests in a single invocation.
Do this by inspecting the current Rake application, rather
than hardcoding a list of tasks to be run (which would need
to be updated when we add a new task to the "test:unit"
namespace).

Other test categories may be formed by using the above strategy
(ie - harcoding a list of tasks to run), but this 'all' task is
intended to run all unit test tasks available.
Silence output from ActiveRecord migrations

... during tests