Update unit testing output for additional test

7898fcf#diff-aa011c62f1a0a519ca875c43c358d6084f87256a4a3ff354d4b0a7eef807e27dR158 added a new test but forgot to update the total number of tests to run.

Author: martin-g

src/testing/unit_testing.md

@@ -155,7 +155,7 @@ Running these tests gives us:
 ```shell
 $ cargo test
 
-running 3 tests
+running 4 tests
 test tests::test_any_panic ... ok
 test tests::test_divide ... ok
 test tests::test_specific_panic ... ok