fixescoder/internal#966

TestCloserStack_Timeout createsasyncClosers which allow control over the exact timing and order of their close method returning. They also, as a final backstop will throw an error if the test context ends before they are unblocked.

TestCloserStack_Timeout unblocks allasyncClosers in a defer and then ends the test. This deferunblocks the running close goroutines, but does not wait for them to finish. Since the test context is canceled as soon as the test completes, this creates a race condition where the close goroutines can trigger the context cancelled arm of theselect statement.

The fix is to both unblock and wait for all close goroutines to complete before ending the test and cancelling the context.