[Draft] Split unit tests in CI

[Akshat-Jain]

Description

Currently, the following unit tests take around 50 minutes to run in our CI pipeline:

• processing_modules_test
• indexing_modules_test
• other_modules_test

We can achieve more parallelism by leveraging more GitHub runners, and splitting the tests across them.

This PR splits the unit tests for each of the above 3 modules based on the first character of the test class name into 2 groups:

• First group: Test classes starting with A-K
• Second group: Test classes starting with L-Z

The results can be seen with the test runs taking half the duration compared to before.

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This PR has:

• been self-reviewed.
• 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.