This mostly removes a lot of repetition by predefining some nodes and
other data structures, plus adds some helpers for creating Peer entries
in the netmap. Several existing test cases were reworked to ensure
better coverage of edge cases, and several new test cases were added to
handle some additional responsibility that is in (or will be shortly
moving in) suggestExitNode().
Updates tailscale/corp#19681
Change-Id: Ie14c2988d7fd482f7d6a877f78525f7788669b85
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
db6447ce63