tka/sync: add regression test for compacted nodes on forked chains

We previously identified sync failures that occur when a node falls behind
the remote, and compacts away most its local state. We fixed the underlying
issue in #19444, but that PR only tested the basic scenario where the
local chain is a direct ancestor of the remote chain.

This patch adds an explicit regression test for the case where a node is on
a fork (that is, its HEAD is not part of the remote's active chain).

Although #19444 happened to cover this case, other proposed patches did not
handle the forked state. Adding this test locks in the behaviour and prevents
future sync regressions in this area.

Also, add a shared helper for writing this sort of TKA sync test.

Updates tailscale/corp#40404

Change-Id: I78fdc6beaf71392edf11806197f126db48886f93
Signed-off-by: Alex Chan <alexc@tailscale.com>