how to split available data into training and testing (Information security)
You should check out k-fold cross validation techique. Its a quality control technique which makes sure, the prediction/classification done by a model would generalize well to unseen test data. The idea is to split the dataset into k times and use it recursively to train and cross validate your network which at end can be used on the test data to report the actual accuracy.

Here is a link for reference : 
https://machinelearningmastery.com/k-fold-cross-validation/