fix tests w/ Devise

The Devise::TestHelper class should only be mixed into the controller
tests.  I misread the info in the Devise README.  This is the way the
test_helper.rb should be.

Before this change, controller tests ran fine, but model tests would
error out with the error, “NoMethodError: undefined method `env' for
nil:NilClass”.
Course validation and basic test

Added a validation on the course duration field to ensure the value can
only be an integer.  There is also now a single unit test for the
Course model, which ensures that the model validation is working (and
also shows that the testing is wired up correctly)