xds test: read ACK to clear req chan (#3266)
xds test: read ACK to clear req chan (#3266)

The real test relies on req chan to know when the fake xds server
receives the request. Without this, req chan may unblock for the ack,
before the real watch handler is ready.
grpc/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go,menghanl/grpc-go,grpc/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go