[FLINK-10750][tests] Harden SocketClientSinkTest

The tests attempts to re-bind a server to a specific port. If the port is taken the test will fail.
The test is now skipped instead if this happens.

This closes apache#7040.

flink-streaming-java/src/test/java/org/apache/flink/streaming/api/functions/sink/SocketClientSinkTest.java

@@ -24,11 +24,13 @@
 import org.apache.flink.util.TestLogger;
 
 import org.apache.commons.io.IOUtils;
+import org.junit.AssumptionViolatedException;
 import org.junit.Test;
 
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStreamReader;
+import java.net.BindException;
 import java.net.ServerSocket;
 import java.net.Socket;
 import java.util.concurrent.Callable;
@@ -279,7 +281,12 @@ public Void call() throws Exception {
  retryLatch.countDown();
 
  // Restart the server
- serverSocket[0] = new ServerSocket(port);
+ try {
+ serverSocket[0] = new ServerSocket(port);
+ } catch (BindException be) {
+ // some other process may be using this port now
+ throw new AssumptionViolatedException("Could not bind server to previous port.", be);
+ }
  Socket socket = serverSocket[0].accept();
 
  BufferedReader reader = new BufferedReader(new InputStreamReader(