Fix illegal reflective access while running tests

[Sineaggi]

Fixes tests from emitting warning when mocking Optional, a final class

> Task :jib-core:test
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.mockito.internal.util.reflection.ReflectionMemberAccessor (file:/Users/cwalker/.gradle/caches/modules-2/files-2.1/org.mockito/mockito-core/4.11.0/ce5226440c2ee78915716d4ce3d10aed2dbf26fb/mockito-core-4.11.0.jar) to constructor java.util.Optional()
WARNING: Please consider reporting this to the maintainers of org.mockito.internal.util.reflection.ReflectionMemberAccessor
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release