Fixes #3826

The cause of this is that mockSingleton implementation overloaded the existing MockingProgress#mockingStarted callback. When using MockitoJUnit rule, UniversalTestListener uses this callback to track created mocks. If the scoped singleton mock is closed, then the testFinished downstream logic would look up mocking details on the singleton which is no longer a mock leading to this exception.

It was probably a mistake to reuse the mockingStarted callback originally - let's mimic the mockStatic path which uses its own dedicated callback for reporting created static mocks.

Note that the unused-stubbing detector stuff (used with "strict stubs" strictness) currently does not support static mocks, and it does not support singleton mocks either. That seems okay; a future PR can add support if desired.

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