Revert "Upgrade to latest minitest."

This reverts commit a4f136dcaec906e8edae06bf313ec27f5d30044c.

We've noticed that the latest version of minitest prevents us from
running focussed (i.e. specified by name) tests. @edds has also
noticed that it's generating some warnings on CI. We're
downgrading until we have time to investigate further.