Move settings fixture into test_assets.py

This fixture is only used in one place, so let's just use it in that one
place.
DictMock is only used in test_api.py
Move tests alongside tested modules

This commit moves our unit tests into 'test' packagaes alongside the
modules they are intended to test. This has a number of advantages:

- predictable test locations: the tests for module foo.bar are always at
  foo.test.bar_test
- 1:1 mapping module to test file: this helps enforce the testing of
  isolated units
- easy overview of module coverage: we can quickly and easily identify
  modules that are not tested
Merge pull request #1682 from hypothesis/unit-tests-with-modules

Move tests alongside tested modules
api: Use the string versions of the client secret

This is ugly, but because Consumer#secret is a UUID, we should be using
the Consumer#client_secret property here which is a string (unicode)
representation of the secret.

The prior state causes 500s on the token endpoint on a fresh checkout.

This commit also adds a test suite for h.api.Token