Convolutional layer dropout layer in keras
Let's clarify few things about dropout. And Neil Slater is to credit for this answer since his comments helped formulate a more clear explanation.

First of all, dropout is a regularization method, it is usually only applied during training (although it can be used in prediction as an approximation to a Bayesian Neural Network as is explained by Yarin Gal's paper). As you might have understood, its goal is to limit overfitting. That is, to train a model that will generalize better to newly unseen data samples. So it has nothing to do with testing but everything to do with training.

Second, the reason why you might have seen the output multiplied by $p$ at prediction time is a trick used with the very basic implementation of dropout referred to as vanilla dropout. At prediction time (or testing time if you prefer this wording) no need to drop anymore, but it is needed to scale the outputs by $p$. The reason is that because at training time, dropout was perform with a probability $p$, then the output need to be scaled to adjust for $p$ at prediction time.

Third, inverted dropout (which is the dropout implementation used in all serious DL libraries) does not need to scale the output at prediction time because scaling by $p$ is actually already performed at training time (by dividing by $p$). Therefore, no need for any trick at prediction time!

Finally, the concept is nicely explained in these videos from the Udacity Deep Learning course or this course from Stanford.

I hope this is clear enough. :)