Added SubManager.append tests

Some findings:
* Because FrameTime is currently the only place where correctness of times is checked, it should
  throw exceptions instead of assertions.
* It is impossible to make some tests with mocks without a proper mocking framework so it might be
  possible to require from a user Python >= 3.3 to run tests in a future. Because of that currently
  test_SubManager doesn't use FrameTimeMocks anymore.