The $advice mock method only works if there is at least one another, usual mock method too.
For example the following test does work. It shows that the mock methods are running. The fn2 mock method at the end has no role in the test. However if I comment out this unnecessary fn2 mock method, the test fails. The output on STDOUT will show that the mock method is not active at all, instead of it, the real fn1 method is called.