Note how databases work during testing

[treyhunner]

@riseriyo and I noticed that this explanation is missing from the workshop tutorials.

1. Tests create a new test database (so there must be access to create this)
2. Tests won't interact with the real database and can't rely on any data in it

[treyhunner]

Relevant documentation: https://docs.djangoproject.com/en/1.5/topics/testing/overview/#the-test-database