Here are all the uses of testimony (Gr. marturia, Strongs 3141) in Revelation:
1:1-2 1 The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants things which must shortly come to pass; and he sent and signified it by his angel unto his servant John:
2 Who bare record of the word of God, and of the testimony of Jesus Christ, and of all things that he saw.
1:9 I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ.
6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
11:7 And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them
12:11 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.
12:17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.
19:10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.
20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
They speak the testimony of Jesus Christ, which is this:
And we have seen and do testify that the Father sent the Son to be the Saviour of the world.
1 John 4:14
It is not talking about the popular Protestant practice of "giving your testimony" where a man talks about how he grew up in a rough home, got into drugs as a teenager, went to prison, and eventually was transformed at a revival altar call. I don't mean to make light of those situations--I think they are often genuine conversion experiences--but I don't think that's what it's talking about (I may very well be wrong!)
It's interesting to note that "testimony" (Gr. marturia Strongs 3141) is similar to "martyr" (Gr. martus Strongs 3144) Of uncertain affinity; a witness (literally [judicially] or figuratively [generally]); by analogy a “martyr”: - martyr, record, witness.