Revert "Revert "DiskWriteThreadPool's destructor is virtual for testing purpose""

This reverts commit deee4145.

Re-instating wrongly reverted commit.

castor/tape/tapeserver/daemon/DiskWriteThreadPool.hpp

@@ -63,7 +63,7 @@ public:
    * Destructor: we suppose the threads are no running (waitThreads() should
    * be called befor destruction unless the threads were not started.
    */
-  ~DiskWriteThreadPool();
+  virtual ~DiskWriteThreadPool();
   
   /**
    * Starts the thread created at construction time.