tstest/integration: add --accept-risk=all to tailscale down

The test would fail if I was running it over SSH.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscale/cli/risks.go

@@ -18,6 +18,7 @@ import (
 var (
 	riskTypes   []string
 	riskLoseSSH = registerRiskType("lose-ssh")
+	riskAll     = registerRiskType("all")
 )
 
 func registerRiskType(riskType string) string {
@@ -35,7 +36,7 @@ func registerAcceptRiskFlag(f *flag.FlagSet, acceptedRisks *string) {
 // risks in acceptedRisks.
 func isRiskAccepted(riskType, acceptedRisks string) bool {
 	for _, r := range strings.Split(acceptedRisks, ",") {
-		if r == riskType {
+		if r == riskType || r == riskAll {
 			return true
 		}
 	}