Remove bitbucket.org from host key verification tests

[MarkEWaite]

Replace bitbucket.org in host key verification test

Test host verifiers with a randomly selected ssh provider.

The bitbucket.org ssh provider is blocking access from the ci.jenkins.io agents. That causes tests to fail. Use other ssh providers.

Reduce the load on ssh providers by only testing one in ten times. Spread the load by using a randomly selected ssh provider instead of always using the same ssh provider.

Also includes other test improvements detected while fixing the failing tests.

• Add mockito-core javaagent for future Java compatibility. Mockito javadoc warns that Java version 21 and later report a warning when trying to use Mockito without the javaagent. Followed the instructions in their documentation to add mockito javaagent.
• Remove invalid Beanstalk URL from test
• Remove test calls to isKubernetesCI() because the method always returns false

Testing done

Automated tests run locally on my computer. Rely on ci.jenkins.io for final verification that the failing tests are fixed.

Submitter checklist

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• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue