net/netcheck: use dnscache.Resolver when resolving DERP IPs

This also adds a bunch of tests for this function to ensure that we're
returning the proper IP(s) in all cases.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0d9d57170dbab5f2bf07abdf78ecd17e0e635399

wgengine/magicsock/magicsock.go

@@ -660,6 +660,7 @@ func NewConn(opts Options) (*Conn, error) {
 		GetSTUNConn6:        func() netcheck.STUNConn { return &c.pconn6 },
 		SkipExternalNetwork: inTest(),
 		PortMapper:          c.portMapper,
+		UseDNSCache:         true,
 	}
 
 	c.ignoreSTUNPackets()