ipn/ipnlocal: evict stale node indexes when a delta upsert replaces a peer

When a node is renamed in the admin console, control sends peers a
single MapResponse delta: a PeersChanged entry carrying the full
updated node with its new Name, and no new DNSConfig (MagicDNS
records are computed client-side from peer names). That arrives as a
NodeMutationUpsert, but nodeBackend's upsert path only added the new
node's index entries and never removed the replaced node's, so
nodeByName retained the old name, and nodeByAddr, nodeByKey, and
nodeByStableID could likewise go stale if those fields changed.

Since 7e609b258 the quad-100 resolver serves MagicDNS answers on
demand from those live indexes, so a renamed peer's old name kept
resolving until something rebuilt the indexes from a full netmap,
such as toggling Tailscale off and on.

Evict the replaced node's index entries before adding the new ones.
Also consolidate the natlab DNS coverage into a single TestMagicDNS
that boots one VM and exercises extra records, search domains, and
peer add/rename/remove end to end, injecting the same MapResponse
shapes that production control sends.

Updates tailscale/corp#45631

Change-Id: I8a418317d930ec8ce112f7bd19bfd5778117a65e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>