Testing meta-issue

This is a master list of unit tests we should add to get better test coverage, specifically negative tests.

**Decommission**
- [ ] Decommission if the resulting state is 3 or more nodes
- [ ] Gracefully handle decommission errors, do not alter cluster if decommission is in the process of happening

**Version Checker**
- [ ] Customer provided image is valid
- [ ] Valid CRDB image
- [ ] Customer can upgrade/downgrade
- [ ] Spins up container and runs `cockroach version`

**Partitioned Update**
- [x] Downgrade to previous version
- [ ] Setting preserved downgrade
- [ ] Fail if we try to upgrade from oldest CRDB version to newest

**Cert Generation**
- [ ] Explicitly test an external cert
- [ ] Good e2e test coverage implicitly