Is a product vision statement worth user testing?

Yes. At some point, you want to have a set of key workflows and key personas nailed as a part of the vision canvas. And using low code or UX mocks etc. - you want to test the validity of the hypothesis to the next granular level. Always be testing, always be validating, always be pivoting and making small tweaks to your plan based on the feedback and data you receive.

Product visions should always be customer-focused. I'm not sure that you need to actually show your vision statement to users to get their feedback, but your vision should be informed by the user problems and product goals that your team hopes to solve over the next few years. In order to do that, you need to know your users intimately, and that often requires customer research and reviewing product data. 

I wouldn't think so. Your vision isn't something that's measurable or track-able in the near time, and hence user testing is hard. Its more likely to want to validate the market feasibility for your vision statement.

Absolutely! How you test your product vision with users is really the question. I don't think I'd ever just put a product vision statement in front of users for feedback. Instead, use your product vision to formulate an imaginary future and build a story around that. Share this story with your users, allowing them to imagine a life where your product vision has come true and build in their minds the journey they took to get there. Ask them how they feel about that future, how their life would be different, and what would they be able to accomplish in that world. They will likely provide amazing and insightful feedback, even if it's not all positive. The key is, your product vision is your guiding light, your foundation, and if you get it wrong everything you do will be wrong too so testing product vision with users (and others) is critical.