test(kafka): Added additional test for kafka startup logic

[benclive]

What this PR does / why we need it:

Adds a test for the processNextFetchesUntilLagHonored function which does quite a lot of logic and I felt was undertested currently.

• The test checks that the retries happen as expected & that kafka is called the expected number of times to replay any pending data.
• I inject the delays by providing a custom time.Since function, which is when retries are determined. The production implementation just uses the real time.Since.
• The test itself is relatively complicated, with several goroutines, so while I think it is correct, I added a timeout context. I'm open to simply closing this PR and skipping this test if the value is not worth the test complexity.

[cyriltovena]

LGTM

Thanks for coming back to this and make sure we have a good test.