When we test a unit A, any change in another unit B or a bug in a different unit should not affect our unit test that is for the unit A. In our case, it was easy because we have only one unit. Later, when we develop the test for the quicksort, we will see that this separation is not that simple.
If the unit tests are properly separated, a failing unit test clearly points out the location of the problem. It is in the unit where the unit test failed. If tests do not separate the units, then a failure in one test may be caused by a bug in a different unit than we expect. In this case, these tests are not really unit tests.
In practice, you should make a balance. If the isolation of the units will be too costly, you can ...
