Doc: Use django.test.TestCase for first testing example

This doc change switches from unittest.TestCase to django.test.TestCase
with some accompanying text changes.

Motivation: you are very likely to get journeyman "drive-by" Django
developers just looking for a simple template example of how to write
a unit test, they will find this canonical example, copy-paste it,
and then get burned because of database coupling.

These type of readers won't read the warning that appears below the
text.