Add a .policy-tests.yml file with tests exercising the policy
that was just landed: the tailcfg/ control-protocol-owners gate,
the "policybot-override:" comment escape hatch (including
defaults-regression guards so the override rule does not
silently accept a normal review or a 👍 comment), and the
always-on "any tailscale/dev review" baseline.
Updates tailscale/corp#13972
Change-Id: I42afb06b0771658c803512cb5de4701450c8a704
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>