Increase timeout in ForcePushDownNestedQueryTest

[kfaraz]

Trying to address some recent flakiness in this new embedded test.
The timeout was recently increased to 20s (from the default 10s), but it still seems to be flaky.

https://github.com/apache/druid/actions/runs/19984107070/job/57315442102?pr=18820#step:6:8061

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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.

[kfaraz]

Might not be needed after all, thanks to the clarification from @abhishekrb19 !