I am currently writing a simple, timer based mini app in C# that performs an action n times every k seconds.
I am trying to adopt a test driven development style, so my
I think what I would do in this case is test the code that actually executes when the timer ticks, rather than the entire sequence. What you really need to decide is whether it is worthwhile for you to test the actual behaviour of the application (for example, if what happens after every tick changes drastically from one tick to another), or whether it is sufficient (that is to say, the action is the same every time) to just test your logic.
Since the timer's behaviour is guaranteed never to change, it's either going to work properly (ie, you've configured it right) or not; it seems to be to be wasted effort to include that in your test if you don't actually need to.