You should separate your base code to a shared (dynamic) library and then write the major part of your unit tests for this library.
Two years ago (2008) I have been involved in large LSB Infrastructure project deployed by The Linux Foundation. One of the aims of this project was to write unit tests for 40.000 functions from the Linux core libraries. In the context of this project we have created the AZOV technology and the basic tool named API Sanity Autotest in order to automatically generate all the tests. You may try to use this tool to generate unit tests for your base library (ies).
