Document how to test ProwJobs locally

[irbekrm]

We have some docs here https://github.com/jetstack/testing#testing-locally but at the moment these instructions are not working (the local cluster gets spun up, but the ProwJob pod always fails- might be because our Prow config uses some deprecated options?

We should look at https://github.com/kubernetes/test-infra/blob/master/prow/build_test_update.md#how-to-test-a-prowjob and add concrete examples how we can use that testing strategy against our ProwJobs.