The testimony would happen via video link, as Snowden is still in exile in the Russian Federation, and the EU and its member countries have pointedly avoided offering asylum to the US whistleblower and fugitive.
According to the BBC, the members of parliament have already contacted Snowden's legal counsel in Russia to se whether he would be willing to testify, but it's too early to tell whether he will accept.
Conservative UK politician Timothy Kirkhope, who's one of the two members of the Justice and Civil Liberties Committee who voted against the motion, has said that he doubts that this testimony will ever happen.
"At least if Snowden wants to give evidence to MEPs he will now have to come out of the shadows and risk his location being discovered. The committee was clear that if Snowden is not willing to appear in an interactive session then the evidence must not go ahead," he added.
This could turn out to be one thing that will prevent Snowden to testify. Also, when he was granted asylum in Russia, Russian president Vladimir Putin explicitly said that he must stop activities aimed at damaging the US.
But, as we know, the leaks haven't stopped, and Snowden has occasionally addressed the public and publicly adressed the Brazil government offering his help against against US spying efforts, and all of this still didn't get him booted out of Russia, so it's doubtful that this testimony would.