tstest/natlab: add a second fake DNS server and split-DNS tests (#20553)

Add a second fake DNS server (4.11.4.12) serving a zone the default one
doesn't, so a split-DNS route can be verified: resolving that name proves the
query was forwarded there.

Tailscale implements a split-DNS route two ways, and a test covers each. When
the OS resolver supports split DNS and every route shares one resolver set,
tailscaled hands the OS that resolver directly and quad-100 stays out of the
query path; TestSplitDNSOSForwarded covers this, reaching the split resolver
over a subnet route as a real deployment would. Otherwise the OS is pointed at
quad-100, which forwards per-domain itself; TestSplitDNS covers this against
both the systemd-resolved and direct backends, along with the other ways a
lookup resolves -- answered locally by quad-100 from an extra record or from the
netmap, and unrouted names still going to the machine's normal resolver.

TestSplitDNSNoMagicDNS covers split DNS on a tailnet with MagicDNS off, which
the two above leave out: one has mixed resolver sets but MagicDNS on, the other
MagicDNS off but a single resolver set. Together those conditions take a
distinct path through compileConfig, where tailscaled reads the OS base
resolver config rather than scoping quad-100 to a match domain.

Both arrangements produce the same answers, so each test also asserts the
guest's resolver state to prove which one it exercised.

Fixes tailscale/corp#44798

Signed-off-by: Brendan Creane <bcreane@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>