Fix skipped charts in check-charts

[timebertt]

How to categorize this PR?

/area dev-productivity
/area quality
/kind bug
/priority normal

What this PR does / why we need it:
Previously, hack/check-charts.sh was only checking charts one level under the given directory (./charts for g/g).
This led to many charts not being checked, that are contained in other directories for structuring (e.g. seed-dns/{entry,provider} or the charts under seed-controlplane).

This PR fixes this behaviour, so that all "top-level" charts will be checked from now on.
It also adds the possibility to add test values in values-test.yaml to the chart directories which will be merged into the default values.yaml. This is useful, if you want to check charts that have a {{ required ... }} statement, but don't want to specify default values.

Checked previously:

charts/cluster-identity
charts/global-network-policies
charts/seed-bootstrap
charts/seed-monitoring
charts/shoot-addons-kyma
charts/shoot-addons
charts/shoot-cloud-config-rbac
charts/shoot-cloud-config
charts/utils-templates
charts/utils-tls-cipher-suites

Checked now:

charts/cluster-identity
charts/garden-project/charts/project-rbac
charts/gardener/controlplane
charts/gardener/gardenlet
charts/global-network-policies
charts/istio/istio-crds
charts/istio/istio-ingress
charts/istio/istio-istiod
charts/istio/istio-proxy-protocol
charts/seed-bootstrap
charts/seed-controlplane/charts/cluster-autoscaler
charts/seed-controlplane/charts/etcd
charts/seed-controlplane/charts/gardener-resource-manager
charts/seed-controlplane/charts/kube-apiserver
charts/seed-controlplane/charts/kube-apiserver-service
charts/seed-controlplane/charts/kube-apiserver-sni
charts/seed-controlplane/charts/kube-controller-manager
charts/seed-controlplane/charts/kube-scheduler
charts/seed-controlplane/charts/network-policies
charts/seed-dns/entry
charts/seed-dns/provider
charts/seed-monitoring
charts/seed-operatingsystemconfig/downloader
charts/seed-operatingsystemconfig/original
charts/shoot-addons
charts/shoot-addons-kyma
charts/shoot-cloud-config
charts/shoot-cloud-config-rbac
charts/shoot-core/components
charts/shoot-core/namespaces
charts/utils-templates
charts/utils-tls-cipher-suites

Which issue(s) this PR fixes:
Ref #2580
Ref #2471 (comment)

Special notes for your reviewer:
/invite @swilen-iwanow

Release note:

It is possible now to add a `values-test.yaml` file to helm charts to specify default values for chart checks, which will be merged into the default `values.yaml` when running `hack/check-charts.sh`. This is useful for the case, that charts have a `{{ required ... }}` statement, but don't specify default values in `values.yaml`.

[swilen-iwanow]

tested it with my PR. works gr8. thanks!

[vpnachev]

/lgtm