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

Dockerfile

@@ -125,6 +125,7 @@ ENV VPNSP=pia \
     LOG_LEVEL=info \
     # Health
     HEALTH_SERVER_ADDRESS=127.0.0.1:9999 \
+    HEALTH_ADDRESS_TO_PING=1.1.1.1 \
     HEALTH_OPENVPN_DURATION_INITIAL=6s \
     HEALTH_OPENVPN_DURATION_ADDITION=5s \
     # DNS over TLS