Migrate the test suite from nose to unittest

[mdickinson]

We should migrate the test suite to use unittest.

Subtasks:

• Move existing function-based tests into unittest.TestCase subclasses
• Update the CI machinery to use unittest
• Update assertions to unittest-style (e.g., self.assertEqual(a, b) instead of assert a == b)

[mdickinson]

There are also several uses of deprecated unittest methods that could be fixed at the same time.