Is a testimony from devil about Jesus, good? Mark 1: 21-24
  They went to Capernaum, and when the Sabbath came,
  Jesus went into the synagogue and began to teach. The people were
  amazed at his teaching, because he taught them as one who had
  authority, not as the teachers of the law. Just then a man in their
  synagogue who was possessed by an impure spirit cried out, “What do
  you want with us, Jesus of Nazareth? Have you come to destroy us? I
  know who you are—the Holy One of God!”


When Jesus was at Capernaum, a man who is possessed with demon came to him and spoke:   


  "....I know who you are—the Holy One of God!" 


But he shut him up.


  Mark 1:25
  “Be quiet!” said Jesus sternly. “Come out of him!” The impure spirit
  shook the man violently and came out of him with a shriek.


And again at evening he again drove away many demons but this time he didn't allow these demons to speak at all.


  Mark 1:32-34
  That evening after sunset the people brought to Jesus all the sick and
  demon-possessed. The whole town gathered at the door, and Jesus healed
  many who had various diseases. He also drove out many demons, but he
  would not let the demons speak because they knew who he was.



Why did Jesus stop the first demon (mentioned here) to talk and not
the others mentioned later?


First demon, was so scared of Jesus and gave a testimony about Jesus(the Holy One of God) in front of everyone.


A testimony from devil about Jesus would have make the people
gathered around him to believe in Jesus more, right?
Rhoads, Dewey and Michie say, in Mark as Story, page 83 (third ed), the demons in Mark's Gospel seek 'to get power over Jesus by naming him as "the holy one of God" and "son of the most high God." These statement might be true, but we can see from that analysis that the were not intended to be "good".

Rhoads, Dewey and Michie (ibid, page 61) say the composer of Mark uses "considerable storytelling skills".  I see one example of those skills to be his ability to present to his first-century audience the portrayal of Jesus as son of God in such a way that early Christians could not be accused by Jews of blasphemy or by the Romans of mocking the emperor, who was regarded as the son of a god. In Mark's Gospel, only outsiders call Jesus the son of God. 

So, when the demons refer to Jesus as "the holy one of God" and "son of the most high God", the Christians could not be faulted, because clearly the demons were opposed to Jesus. Even when Jesus does not allow the demons to speak "because they knew who he was", Mark's audience knew this was further, unspoken evidence that Jesus is truly the Son of God. When the High Priest raises this topic by asking (Mark 14:61), "Art thou the Son of the Blessed?" the Christians remain blameless. The centurion watching the crucifixion says, "Truly this man was the Son of God," a sign of approval from a Roman of some importance.  God himself describes Jesus as his beloved son (Mark 1:11; 9:7), and God is above criticism.

Testimony from demons is powerful, because they knew the truth about Jesus, even if they were the embodiment of first-century evil. Mark's author has made good use of their testimony to ensure that Jesus was properly recognised as the holy one of God, the Son of God.  The later gospels were less constrained, so we begin to see Jesus directly referred to as the Son of God.