mojo: Fix python unit tests.

One of the python unit test suites doesn't actually derive from
mojo_unittests.MojoTestCase, and thus doesn't do the correct
initialization to make sure that it can use the python mojom machinery
before it actually uses it. This makes the suite derive from the correct
base test case.

BUG=none
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/675573003