.github,.policy-tests.yml: test .policy.yml in CI

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>

internal/tooldeps/tooldeps.go

@@ -10,5 +10,6 @@ package tooldeps
 import (
 	_ "github.com/golangci/golangci-lint/cmd/golangci-lint"
 	_ "github.com/tailscale/depaware/depaware"
+	_ "github.com/tailscale/policybottest"
 	_ "golang.org/x/tools/cmd/goimports"
 )