cmd/k8s-operator/e2e: add tests to cover DNSConfig (#20861)

Deploy k8s nameserver during e2e test setup, and point the cluster resolver
(CoreDNS or kube-dns) to it so tests can resolve MagicDNS names inside the
cluster.

Add a test to verify singleton L7 Ingress is reachable from inside the
cluster using its MagicDNS Name.

Update existing egress tests to use a dedicated tailnet target per test (to
avoid conflicts). Egress tests now also verify that an egress target is
reachable from within the cluster using both its Service and MagicDNS name.

To successfully curl using the target's MagicDNS name, publish test CAs as a
ConfigMap to the cluster, and mount these for each curl pod.

Fixes tailscale/corp#38027

Signed-off-by: Becky Pauley <becky@tailscale.com>