interop/client: simplify fallback test client and parameterize fallback deadline (#5248)
interop/client: simplify fallback test client and parameterize fallback deadline (#5248)


grpc/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,grpc/grpc-go,dfawley/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go