Feat: HEALTH_ADDRESS_TO_PING variable

- Defaults to `1.1.1.1`
- Add more Ping integration tests with different addresses
- Add unit test pinging 127.0.0.1
- Add comment explaining why we need to use ICMP instead of UDP

internal/configuration/health.go

@@ -12,6 +12,7 @@ import (
 // Health contains settings for the healthcheck and health server.
 type Health struct {
 	ServerAddress string
+	AddressToPing string
 	VPN           HealthyWait
 }
 
@@ -24,6 +25,8 @@ func (settings *Health) lines() (lines []string) {
 
 	lines = append(lines, indent+lastIndent+"Server address: "+settings.ServerAddress)
 
+	lines = append(lines, indent+lastIndent+"Address to ping: "+settings.AddressToPing)
+
 	lines = append(lines, indent+lastIndent+"VPN:")
 	for _, line := range settings.VPN.lines() {
 		lines = append(lines, indent+indent+line)
@@ -49,6 +52,11 @@ func (settings *Health) read(r reader) (err error) {
 		return fmt.Errorf("environment variable HEALTH_SERVER_ADDRESS: %w", err)
 	}
 
+	settings.AddressToPing, err = r.env.Get("HEALTH_ADDRESS_TO_PING", params.Default("1.1.1.1"))
+	if err != nil {
+		return fmt.Errorf("environment variable HEALTH_ADDRESS_TO_PING: %w", err)
+	}
+
 	retroKeyOption := params.RetroKeys([]string{"HEALTH_OPENVPN_DURATION_INITIAL"}, r.onRetroActive)
 	settings.VPN.Initial, err = r.env.Duration("HEALTH_VPN_DURATION_INITIAL", params.Default("6s"), retroKeyOption)
 	if err != nil {