@Beta
  public void onTimeout(Closure<String> timeoutMessage) {
    this.timeoutMessage = timeoutMessage;
  }
Sets the closure that is evaluated when a timeout is reached.
<p>
The closure can use a {@link Throwable} as an input parameter,
which is thrown by the test conditions when a timeout is reached. The result of this
closure is added to the {@link SpockTimeoutError} message. Calling it with null resets the timeout message.

@param timeoutMessage the closure that is evaluated when a timeout is reached
@since 2.4
spock-core/src/main/java/spock/util/concurrent/PollingConditions.java