You could always use something like chromatic or applitools.
In my opinion, I think you should consider this approach:
Use Phaser minimally. All the actual game logic runs in a webWorker, and Phaser is literally only used to display things.
That way, ideally, you don’t even need Phaser to play your game and a text-only version is possible.
Caveats: this probably works best for ‘slow’ games like turn-based JRPGs or turn-based strategy games.