Is it correct to join training and validation set before inferring on test-set?
Yes, once you optimized your model and parameters with the validation set, it is advised to train your final model on the combination of the training and validation sets before applying it to the test set. Indeed, you can see the validation set as a subset of the training set. It is used to tweak your models and your parameters, but once it is done, it would just be a waste to not use the validation set for training during the testing phase.

Remember that once you tested your model on the test set, you should not tweak it anymore.