Add config-features.yaml into test/config (#8140)
Add config-features.yaml into test/config (#8140)

The latest net-istio especially istio-webhook depends on
`config-feature` configmap.

Then the upgrade tests uses the latest net-istio, but `config-feature`
configmap was not included in the previous version, so the istio-webhook
fails to run.

To fix it, this patch adds config-features.yaml into test/config.
knative/serving,knative/serving