You don't really say what your general knowledge is about undergraduate mathematics. You could certainly have deep knowledge without taking courses, of course. But if you have only the knowledge given in the two courses you mention and are simply contemplating taking the GRE subject test in Mathematics, I would caution against it.
The test is very difficult and very broad. It is broad since undergraduate programs differ and the test developers want to make it possible for everyone to find something in the test that they know deeply. However, that also means that even a student with a good undergraduate mathematics degree will find many questions on the exam about which they know nothing at all. Even the terminology of the question is foreign. It isn't like a ramped up SAT exam. But without a background in at least Analysis, Abstract Algebra, and Topology it will be very difficult to pass.
On the other hand, if you have already taken the test and have achieved a high score, I would personally mention it. It demonstrates a high level of abstract thinking that will serve you well in anything. Since Economics can be (but isn't necessarily) fairly computational you won't hurt yourself by saying it, and might help yourself. Likewise, the ability to easily understand economic models is aided by a mathematics background.