fix: remove use of testing.T.FailNow() inside goroutine

[alrs]

This fixes TestClientAutoRefreshShutdownRace() to not fire t.Fatal() inside a goroutine.

t.Fatal() is a convenience function that calls t.Log() and t.FailNow(). t.FailNow() should not be called from inside a goroutine. From the godoc:

FailNow must be called from the goroutine running the test or benchmark function, not from other goroutines created during the test.

This replaces the done channel with an error channel, and tests the error variable outside the goroutine.

[alrs]

I'm happy to fill out a CLA, should someone provide a link to one.

[bai]

Here we go, it's actually one of the failing checks: https://github.com/Shopify/sarama/pull/1601/checks?check_run_id=428029640 👀

[alrs]

@bai CLA is complete.

[bai]

Thanks for contributing.