Fix race in tests

[sharonjl]

Run tests with the -race flag:

go test -v -race ./...

[u5surf]

@sharonjl
Hi, I fixed these tests race conditions using sync.Mutex lock for consumeCount.
On the other hand, I think it is better to use channel lock than using Mutex following golang philosophy.
https://blog.golang.org/share-memory-by-communicating
which should I use channel or Mutex?
c.f. https://github.com/golang/go/wiki/MutexOrChannel

[sharonjl]

Oh awesome, honestly channels might be a bit overkill. These are just tests, as long as they work properly and test the functional aspect of the library, either is fine. I'll leave it up to you!