MINOR: Use Mockito's strict stubs functionality for Connect tests and cleanup unused stubs

[yashmayya]

• While migrating tests from EasyMock / PowerMock to Mockito (https://issues.apache.org/jira/browse/KAFKA-7438), we've been using Mockito's strict stubs functionality to help keep tests clean.
• This patch applies the strict stubs rule to the rest of the Connect tests and cleans up unused stubs.
• One notable test that has been skipped is the ExactlyOnceWorkerSourceTaskTest. This is because it is already using a separate JUnit runner and enabling strict stubs via the MockitoJUnit rule causes UnnecessaryStubbingException to be thrown even with unused stubs in the @Before method (i.e. any stubs setup there need to be used by every test). This doesn't occur when using the MockitoJUnitRunner; from the docs here -

Mockito JUnit Runner triggers UnnecessaryStubbingException only when none of the test methods use the stubbings. This means that it is ok to put default stubbing in a 'setup' method or in test class constructor. That default stubbing needs to be used at least once by one of the test methods.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[yashmayya]

@C0urante @mimaison would either of you be able to review this minor cleanup?

[mimaison]

Thanks, LGTM

[yashmayya]

Thanks Mickael!