Introduce a flavor of the IntegrationTestMixin that doesn't use Mockito

[cacoco]

It would be great to have a version of the IntegrationTestMixin that does not rely on Mockito allowing users to "bring their own mocking" to the table. This would help in the cases where people would like to use a newer version of Mockito that the framework currently supports or some other mocking framework altogether.

This would most likely entail moving the mocking functionality to another trait and figuring out the best way to delay mixing it in until where necessary to maintain backward compatibility.

Along those lines, we generally ask that users mix in the com.twitter.inject.Mockito trait when mocking so we may have an avenue in which we can shift around responsibility and maintain backward compatibility by moving the mocking functionality currently in the IntegrationTestMixin into the com.twitter.inject.Mockito trait itself.

Then users can choose to use the IntegrationTestMixin and its subclasses (which would be decoupled from any mocking) with or without the Mockito trait.

[cacoco]

After 841f697 the IntegrationTestMixin no longer has any mocking dependency and users should be able to "bring their own" mocking to their tests.