Where are the tests?
Essentially every module in Backdrop has a tests directory. So, for example, if you look at the node module structure; you'll see this:
BACKDROP_ROOT - core - modules - node - . - . - . - tests - node.test - node.tests.info
What are the tests?
The tests are classes that include a setup function and some testing functions.
If you open up the node.test file you can look at some tests. The main thing I keep in my mind when writing tests is the word assert because many of the tests assert that something is TRUE or not true (FALSE) or some other condition.
Let's look at a test.