CI: Enforce that `limbo.json` is up-to-date

[woodruffw]

We shouldn't allow a PR merge unless limbo.json is fully up-to-date. The easiest way to do this is probably to run make limbo.json in the CI and make sure that the test IDs fully match the checked-in copy.

[woodruffw]

See #149 for rationale.

[woodruffw]

This bit us again with #436.