Improve FTPS server shutdown to prevent port conflicts

Added more robust shutdown logic in AbstractFtpsProviderTestCase.tearDownClass():
- Wait 300ms after suspend() to allow active connections to finish
- Increased retry count from 10 to 20 for server stop wait
- Clear all static fields (socketPort, connectionUri) after shutdown
- Increased final wait from 500ms to 1000ms to ensure port is fully released

This helps prevent issues when running multiple FTPS test classes sequentially
in the same JVM, though the tests still work best when run in separate forks.

Author: gnodet

commons-vfs2/src/test/java/org/apache/commons/vfs2/provider/ftps/AbstractFtpsProviderTestCase.java

@@ -154,10 +154,22 @@ synchronized static void setUpClass(final boolean implicit) throws FtpException
      */
     synchronized static void tearDownClass() {
         if (embeddedFtpServer != null) {
+            // First suspend to stop accepting new connections
             embeddedFtpServer.suspend();
+
+            // Wait a bit for active connections to finish
+            try {
+                Thread.sleep(300);
+            } catch (final InterruptedException e) {
+                e.printStackTrace();
+            }
+
+            // Now stop the server
             embeddedFtpServer.stop();
             Thread.yield();
-            int count = 10;
+
+            // Wait for server to fully stop
+            int count = 20; // Increased from 10 to 20
             while (count-- > 0 && !embeddedFtpServer.isStopped()) {
                 final int millis = 200;
                 System.out.println(String.format("Waiting %,d milliseconds for %s to stop", millis, embeddedFtpServer));
@@ -167,7 +179,18 @@ synchronized static void tearDownClass() {
                     e.printStackTrace();
                 }
             }
+
+            // Clear the reference
             embeddedFtpServer = null;
+            socketPort = 0;
+            connectionUri = null;
+
+            // Additional wait to ensure port is fully released and all resources cleaned up
+            try {
+                Thread.sleep(1000); // Increased from 500 to 1000
+            } catch (final InterruptedException e) {
+                e.printStackTrace();
+            }
         }
     }