How to represent a document in test data with the Document-Term Matrix created from the training set?
The easiest way is to treat all out-of-vocabulary terms as a specific term in your matrix (i.e. "OOV").

So for instance, if my training data contains 3 words: "I", "like", "cake", my document-term matrix would contain 4 items, "I", "like", "cake", and "OOV".