testdata: get test assets related to package root instead of GOPATH (#2272)
testdata: get test assets related to package root instead of GOPATH (#2272)

fixes #2270
dfawley/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go,thinkerou/grpc-go,improbable-io/grpc-go,thinkerou/grpc-go,dfawley/grpc-go,grpc/grpc-go,improbable-io/grpc-go,menghanl/grpc-go