🐞 Eliminate flaky test failures

Integration tests would fail approximately 11% of the time (4/35) when
run in parallel (on my 8-core MacBook Air). The fix was to lengthen the
amount of time (1ms → 2ms) a port was held to make sure it was really,
truly free. After change, the tests ran 32 times without a failure.

Fixes, during `ginkgo -r -p --until-it-fails .`
```
I couldn't bind to any IPs on port 1974, so I'm exiting
...
  Waiting for:
      Ready to answer queries
  In [JustBeforeEach] at: /Volumes/workspace/sslip.io/src/sslip.io-dns-server/integration_flags_test.go:28 @ 11/11/22 10:38:02.045
```

src/sslip.io-dns-server/integration_test.go

@@ -486,7 +486,7 @@ func isPortFree(port int) bool {
 	// we must Sleep() in order to avoid a race condition when tests
 	// are run in parallel (`ginkgo -p`) and the `ListenUDP()` and `Close()`
 	// we sleep for a millisecond because the port is randomized based on the millisecond.
-	time.Sleep(1 * time.Millisecond)
+	time.Sleep(2 * time.Millisecond)
 	err = conn.Close()
 	if err != nil {
 		log.Printf("I couldn't close port %d", port)