In order to do proper unit test you may have to simulate other system modules.
Testing seldom will save testing resources, but increase the average time of finding bugs.
Testing frequently will allow finding problems early.
This will save development time for 2 reasons:
(draw feedback loops).
One common scenario is when you need to start work with old or recent version. You take the version into your computer and start adding new features. Then you try to make the code run properly and you encounter problems. After a long debug time, you find out that the problem is not with your code, but with the base version that you used. It might not work on recent operating system or with new release of development environment, or you just took the wrong version. Next time it is better to do a small test to ensure that the base version is OK.
