Refactor tests and code clean up

[abhishekrb19]

Summary of changes in this patch by file:

Changes in TestDerbyConnector:

• Introduce a static wrapper class SegmentsTable, which provides utilities for commonly used SQL commands for tests in update() and updateUsedStatusLastUpdated().
• Clean up references in tests to use the above new methods where applicable. For example, see usages in SqlSegmentsMetadataManagerTest, IndexerSQLMetadataStorageCoordinatorTest, SQLMetadataConnectorTest.

Changes in KillUnusedSegmentsTaskTest:

• Introduce a builder class KillUnusedSegmentsTaskBuilder and refactor usages.
• Move common segment creation to setup() method.
• Replace usages of Thread.sleep() with calls to the new updateSegmentsTable() method.
• Miscellaneous indentation adjustments based on review feedback from Kill segments by versions #15994.

Change in IndexerSQLMetadataStorageCoordinator:

• Refactor some code in favor of the verifySegmentsToCommit() method.

This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.

[kfaraz]

Thanks for the clean up, @abhishekrb19 !
I have left minor suggestions but they are mostly optional.