derp: add test of client peerPresent parsing across server versions

Test that the client parses peerPresent frames from servers of various
eras: old servers that send fewer fields than the client knows about,
and newer servers that send trailing fields the client doesn't know
about, which it must ignore. This matters during rollouts of new DERP
servers, when a region's meshed nodes and watchers run a mix of
versions.

This is in advance of a following commit that extends the frame with a
new trailing field.

Updates tailscale/corp#24454

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I0d46126a6003a9eb07a4da46dc291872149b1a07