Fix flaky test in RetryableS3OutputStreamTest

[Akshat-Jain]

Description

As part of #16481, we have started uploading the chunks in parallel. That means that it's not necessary for the part that was uploaded at the last to be less than or equal to the chunkSize (as the final part could've been uploaded earlier).

This made a test in RetryableS3OutputStreamTest flaky where we were asserting that the final part should be smaller than chunk size.

This PR fixes the test, and also adds another test where the file size is such that all chunk sizes would be of equal size.

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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.