fix(test): consumer group fetch request messages

[stsmurf]

There is a bug in the tests with

NewMockSequence(
    NewMockFetchResponse(t, 1).
        SetMessage("my-topic", 0, 0, StringEncoder("foo")).
        SetMessage("my-topic", 0, 1, StringEncoder("bar")),
    NewMockFetchResponse(t, 1),
)

This sets the message on the first NewMockFetchResponse twice and does not set the message on the second response. This was covered up in the test because of the break in the for loop over claim.Messages(). The update now passes the messages through a channel back to the test for assertion.

In addition, the context.WithCancel was not having any affect on the test. Closing the group causes the context to be Done. This is verified by utilizing the same channel.

[dnwe]

Thanks! Good spot