Test: Remove mock LoadQueuePeon from coordinator unit tests

[kfaraz]

Changes

• Simplify DruidCluster.Builder
• Merge BroadcastDistributionRuleSerdeTest into BroadcastDistributionRuleTest
• Refactor UnloadUnusedSegmentsTest and other tests to avoid mocking DruidServer and LoadQueuePeon objects.
• Add abstract test class CoordinatorBaseTest.

This is part of a bigger PR which refactors RunRulesTest as well but as that was becoming too cumbersome, I have broken it up into smaller parts.

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This PR has:

• [ ] been self-reviewed.
  • [ ] using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• [ ] added documentation for new or modified features or behaviors.
• [ ] a release note entry in the PR description.
• [ ] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• [ ] added or updated version, license, or notice information in licenses.yaml
• [ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [ ] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• [ ] added integration tests.
• [ ] been tested in a test Druid cluster.

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