Rework SOCKS5 unit tests so that they can be run in parallel

As result it is now also possible to start multiple local SOCKS5
proxies with different port, which is usually not necessary in real
life but useful for unit tests.

smack-core/src/test/java/org/jivesoftware/smack/util/NetworkUtil.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2016 Florian Schmaus
+ * Copyright 2016-2019 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,7 @@ public class NetworkUtil {
 
     private static final Logger LOGGER = Logger.getLogger(NetworkUtil.class.getName());
 
-    public static ServerSocket getSocketOnLoopback() {
+    public static ServerSocket getSocketOnLoopback() throws IOException {
         final InetAddress loopbackAddress = InetAddress.getLoopbackAddress();
         final int portMin = 1024;
         final int portMax = (1 << 16) - 1;
@@ -40,13 +40,12 @@ public class NetworkUtil {
                 break;
             } catch (BindException e) {
                 LOGGER.log(Level.FINEST, "Could not bind port " + port + ", trying next", e);
-            } catch (IOException e) {
-                throw new IllegalStateException(e);
             }
         }
 
         if (serverSocket == null) {
-            throw new IllegalStateException();
+            throw new IOException("Could not bind any port between " + portMin + " and " + portMax
+                            + " on loopback address" + loopbackAddress);
         }
 
         return serverSocket;