Testamento literally means "will" or "testament," the legal document for your belongings after death. It's a very dramatic and unusual word to find in a pop song!
Becky G uses it metaphorically to express the depth of her heartbreak. She sings, "El día que terminamos firmé mi testamento" (The day we broke up, I signed my will), to show that the breakup felt like a death and that her heart is now "muerto en vida" (dead while alive).
Ever had a moment you wish you could live in forever? That's the heart of this duet by Becky G and Oscar Ortiz. They sing from the perspective of two people who have broken up but are filled with regret. They were so close to making it work: "Casi logramos que funcionara esto / Casi, pero no" (We almost made this work / Almost, but no). The whole song is a fantasy about turning back time, with the central, heartbreaking idea being "Si hubiera sabido que era el último beso / Todavía estaría besándote" (If I had known it was the last kiss / I'd still be kissing you).
What makes this song so intense is that both singers feel the exact same way. They trade verses about how they would do anything to go back and hold on tighter. The lyrics get pretty dramatic, with one of them claiming "El día que terminamos firmé mi testamento / Porque está muerto en vida ya mi corazón" (The day we broke up I signed my will / Because my heart is already dead in life). They promise to love each other even after "diez mil años" (ten thousand years), holding on to the illusion that they might see each other again.