Configure ByteBuddy Java agent in test tasks

[bclozel]

While tests don't generally fail on the CI, we should configure the ByteBuddy java agent in our test tasks as dynamic agent loading is not allowed anymore as of Java 21. This should cover both Mockito and mockk usage in our tests.

[bclozel]

Reopening to replace the Mockito agent configuration with the generic ByteBuddy agent instead to cover for both Mockito and mockk.