appc: TestTCPSNIHandler flake

Flake https://github.com/tailscale/tailscale/actions/runs/6717340109/job/18255027051?pr=10042

```
=== RUN   TestTCPSNIHandler
    handlers_test.go:154: EOF
--- FAIL: TestTCPSNIHandler (0.00s)
FAIL	tailscale.com/appc
```

/cc @raggi @catzkorn 