Update Implementation report link / verify tests

[bkardell]

Currently we have an implementation report listed which uses a query on wpt.fyi. At some point it started generating some false positives thanks to some JS apis, and now it's getting others thanks to CSS ones. The web-dx group has been going through and mapping features.. It's a lot like what we do for interop scoring in general. I'm not 100% sure how to query it yet, but I'm working on it -- meanwhile we should work to verify/improve https://github.com/web-platform-dx/web-features/blob/main/features/mathml.yml and then update the spec link accordingly.