(1) God's testimony converts us; "turns" us; in the OT it's most often paired with Torah. Very action-oriented: guarding God's testimony means keeping torah: law or teaching broadly construed.
(a) The torah of YHWH is perfect, converting [turning] the soul; the
testimony [Hebrew idah, Greek marturia] of YHWH is trustworthy,
making wise the simple. (Psalm 19:7)
(b) He established testimony for Jacob and appointed torah in Israel, which he commanded our forefathers to teach their children. (Psalm 78:5)
(c) Preserve my life according to your steadfast love, so that I may guard [keep, obey] the testimony of your mouth (Psalm 119:88)
(2) New Testament marturia most often means SPEAKING about things; like courtroom testimony. But actions are also sometimes referred to as "giving testimony" e.g. Mark 6:11 "shake the dust off your feet when you leave, as a testimony against them." Testimony and testifying, witnessing and being a witness: these are virtually interchangeable concepts. Evidence and actions "speak" as well as words.
(a) John 1:7 He came as a witness (marturia) to testify (martureo)
to the light, so that all might believe through him.
(b) Matthew 23:23-31 Woe unto you, scribes and Pharisees, hypocrites! For you tithe mint, dill, and cummin, and have neglected the weightier matters of the law: justice and mercy and faith . Thus you testify against yourselves that you are descendants of those who murdered the prophets.
(3) A person's testimony is a product of God's testimony to and through that person.
(a) 1 John 5:10 Those who believe in the Son of God have the testimony (marturia)
in their hearts. Those who do not believe in God have made him a liar by not
believing in the testimony (marturia) that God has testified (martureo)
concerning his Son.
(b) Acts 14:3 So they remained for a long time, speaking boldly for the Lord, who testified to the word of his grace by granting signs and wonders to be done through them.
(c) Hebrews 10:15-26 And the Holy Spirit also testifies to us about this, saying I will put my laws in their hearts, and I will write them on their minds.
(4) The word "testimonies" only occurs in the KJV as the plural of "laws" or "decrees" and in one place (Mark 14:56) as the plural of courtroom testimony: "For many testified falsely against him, but their testimonies did not agree." There is no biblical sense of "testimony" as a pattern of action or way of life that "witnesses" over a period of time. The same applies to early Friends' writings.
George Fox (Letter 264, 1669) "And such testimonies of Friends as are deceased, let them be recorded, that so the testimony of the Lord through his servants may not be lost, that he raised up in his people, for the same they were carried on through great tribulations and sufferings; and many laid down their lives, and had their goods spoiled, and they persecuted to death, to keep up their testimony, (which the Lord moved them to speak by his spirit,) to future generations. So that those words, that they were moved to speak forth by the power, may not be lost; that the power and spirit of the Lord may be exalted."
(5) The testimony of early Christians and early Friends often led to suffering and even death; hence the etymological relationship between the English word "martyr" and the Greek martureo, to testify or witness.
(a) 2 Timothy 1:8 Do not be ashamed, then, of the testimony about our Lord,
or of me [Paul], his prisoner; but join with me in suffering for the gospel,
relying on the power of God.
(b) Mark 13:9 As for yourselves, beware; for they will hand you over to councils, and you will be beaten in synagogues, and you will stand before governors and kings because of me, as a testimony to them.
Additional reading: Psalm 119; John 1