Test certificates expiring in <2y

[NightTsarina]

Hi,

A very minor issue: the CA certificates used for testing are due to expire in May 2026, and so it would be good to re-generate them in advance, to avoid build failures in frozen releases.

cluster/testdata/certs/ca.pem: 
        Validity
            Not Before: May  5 16:16:00 2021 GMT
            Not After : May  4 16:16:00 2026 GMT

This is mostly relevant to distros like Debian, where we ensure that all packages can be rebuilt from source during a release's support period, but it would be nice not to have to patch this.

Original bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1078634