Don't call t.FailNow in goroutines started by tests.
Don't call t.FailNow in goroutines started by tests.

The docs at https://golang.org/pkg/testing/#T.FailNow say:

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

(Fatalf is documented as "Fatalf is equivalent to Logf followed by
FailNow.")

This was manifesting itself as a race with two concurrently failing
goroutines in my other CL, masking whatever the real problem was.

andreimatei/grpc-go,bradfitz/grpc-go,grpc/grpc-go,dsymonds/grpc-go,peter-edge/grpc-go,iamqizhao/grpc-go,hugoz1234/grpc-go,hugoz1234/grpc-go,bradfitz/grpc-go,improbable-io/grpc-go,andreimatei/grpc-go,MakMukhi/grpc-go,stanley-cheung/grpc-go,improbable-io/grpc-go,dfawley/grpc-go,mpl/grpc-go,thinkerou/grpc-go,fuchsia-mirror/third_party-grpc-grpc-go,rakyll/grpc-go,go4org/grpc,menghanl/grpc-go,grpc/grpc-go,iamqizhao/grpc-go,go4org/grpc,peter-edge/grpc-go,MakMukhi/grpc-go,dsymonds/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go,fuchsia-mirror/third_party-grpc-grpc-go,menghanl/grpc-go,mpl/grpc-go,rakyll/grpc-go,thinkerou/grpc-go