internal: fix flaky test KeepaliveClientStaysHealthyWithResponsiveServer (#4427)
internal: fix flaky test KeepaliveClientStaysHealthyWithResponsiveServer (#4427)

Server should allow `NoStream`, otherwise there's a small chance
(5/1000) the connection will be closed due to `too many pings`.
grpc/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,menghanl/grpc-go,dfawley/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go