This patch adds an integration test for Tailnet Lock, checking that a node can't
talk to peers in the tailnet until it becomes signed.
This patch also introduces a new package `tstest/tkatest`, which has some helpers
for constructing a mock control server that responds to TKA requests. This allows
us to reduce boilerplate in the IPN tests.
Updates tailscale/corp#33599
Signed-off-by: Alex Chan <alexc@tailscale.com>
Alex Chan