Add `test:compat` to CI

[mdrichardson]

Related: #3628 (comment)

Looks like it hasn't been updated in awhile, so it needs to be updated and also added to CI so that it stays up-to-date.

[joshgummersall]

I think test:compat is a command across many packages. Perhaps it can be extracted to the root package.json file and run across all packages? The CI job ought to operate just like the test:schemas command in that it should run the command and fail if there's a diff detected.

[mdrichardson]

@joshgummersall Will do!