Usually, you'd spin up a cluster with the version of k8s on minikube for this task and ensure that the tests pass, following the instructions in the README for https://gerrit.wikimedia.org/g/labs/tools/maintain-kubeusers/+/refs/heads/master
If they don't pass, you'll need to check the errors and changelogs for 1.18 to find out why. It's usually api version changes, but it can be the python-kubernetes client getting too old (it is out of date as is).
If it cannot be run on 1.18 because it is too old, you can validate against 1.19 usually since anything that works on 1.19 should work on 1.18. Deploying to toolsbeta and creating a new tool there (which is only possible for WMCS staff at this time) is the final confirmation before building the image with the :latest tag and deploying to tools.