Although the current version passes or fails on the correct cases, it does so by underflowing when there is a leak because the operands are reversed in a subtraction of after / before bytes allocated.
Update step_4_leak_test_2.rs to fix the order of operands in the leaked bytes check. Previously, this would fail when expected, but by an underflow error from subtracting a larger unsigned number rather than the assertion for the value being zero.