Unit tests give us a way to check code against a specific test case, but it is limited by the creativity and patience of the programmer who writes them. If the function takes a boolean a parameter then it is of course possible to try both 'true' and 'false' and make sure that the function returns the correct result.
However there are many cases where the possible input domain is more complex. For example we may wish to determine that some operations are communicative in nature. In this case we can establish the property that F(L) =F(L') where L' is a list of the same elements as L but in a different order.