In no way do covenant and testament have the same meaning. *Correction of previous post*, diathéké in Greek can be use to reference covenant, and testament but even then each word has its own meaning.
diathéké: testament, will, covenant
Original Word: διαθήκη, ης, ἡ
Part of Speech: Noun, Feminine
Transliteration: diathéké
Phonetic Spelling: (dee-ath-ay'-kay)
Short Definition: a covenant, will, testament
Definition: (a) a covenant between two parties, (b) (the ordinary, everyday sense [found a countless number of times in papyri]) a will, testament.
HELPS Word-studies
1242 diathḗkē (from 1223 /diá, "thoroughly," intensifying 5087 /títhēmi, "place, set") – properly, a set-agreement having complete terms determined by the initiating party, which also are fully affirmed by the one entering the agreement.
Thayer's Greek Lexicon
STRONGS NT 1242: διαθήκη
διαθήκη, διαθήκης, ἡ (διατίθημι);
1. a disposition, arrangement, of any sort, which one wishes to be valid (German Verordnung, Willensverfugung): Galatians 3:15, where under the name ofa man's disposition is meant specifically a testament, so far forth as it is a specimen and example of that disposition (cf. Meyer or Lightfoot at the passage); especially the last disposal which one makes of his earthly possessions after his death, a testament or will (so in Greek writings from (Aristophanes), Plato, legg. 11, p. 922 c. following down): Hebrews 9:16f
2. a compact, covenant (Aristophanes av. 440), very often in the Scriptures for בְּרִית (Vulg.testamentum). For the word covenant is used to denote the close relationship which God entered into, first with Noah (Genesis 6:18; Genesis 9:9ff (cf. Sir. 44:18)), then with Abraham, Isaac and Jacob and their posterity (Leviticus 26:42 (cf. 2 Macc. 1:2)), but especially with Abraham (Genesis 15 and Genesis 17), and afterward through Moses with the people of Israel (Exodus 24; Deuteronomy 5:2; Deuteronomy 28:69 ).