Address issue with flaky test

[MartinWheelerMT]

What

• Move SendContinueHandlerFailure test to its own class.
• Move now shared methods into base class.
• Add TestConfiguration to new test class to ensure that the mock is fully created before wiring.

Why

To address a thread-safety issue in the way Mockito handles mocks:

Is Mockito thread-safe?

For healthy scenarios Mockito plays nicely with threads. For instance, you can run tests in parallel to speed up the build. Also, you can let multiple threads call methods on a shared mock to test in concurrent conditions.

However Mockito is only thread-safe in healthy tests, that is tests without multiple threads stubbing/verifying a shared mock. Stubbing or verification of a shared mock from different threads is NOT the proper way of testing because it will always lead to intermittent behavior. In general, mutable state + assertions in multi-threaded environment lead to random results. If you do stub/verify a shared mock across threads you will face occasional exceptions like: WrongTypeOfReturnValue, etc.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users
• A corresponding change has been made to the Mapping Documentation repository

[github-actions]

Looks good. No mutations were possible for these changes.
See https://pitest.org