There are many tools that astronomers should know about, but that are not astronomy specific. This aims to be a list of such tools, and, by its nature, will certainly be incomplete.
The basic idea of unit testing is to verify that a piece of code works the way it is supposed to work. That is, have the code try to solve the a problem with a known answer, and if the code does not give the right answer, it does not work. There is a module for doing this in python called unittest (or, alternatively, PyUnit).