/** 
 * Resets {@link #context} to {@code null} and clears the {@link ThreadLocal} which was holding that context.
 * This method is automatically invoked by JUnit after each test, but can also be invoked explicitely before
 * to create a new context. It is safe to invoke this method more than once.
 */
@After public final void clearContext(){
  assertSame("Unexpected context. Is this method invoked from the right thread?",context,Context.current());
  if (context != null) {
    context.finish();
    context=null;
  }
}