extend TopGroups.merge test coverage [LUCENE-9010]

This ~~sub~~-task proposes to add test coverage for the `TopGroups.merge` method, separately from but as preparation for #10039 fixing the 'maxScore is sometimes missing' bug.

edit: converted from #10039 bug-fix sub-task to test-type task of its own so that its life can extend beyond the #10039 bug-fix ticket's life for further test coverage extensions

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Migrated from [LUCENE-9010](https://issues.apache.org/jira/browse/LUCENE-9010) by Christine Poerschke (@cpoerschke), updated Nov 12 2019
Attachments: [LUCENE-9010.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-9010/LUCENE-9010.patch) (versions: 2)
Linked issues:
- #10039
- #10084

Start by locating the TopGroups.merge method and its existing tests in the Lucene repository, then review the linked #10039 and #10084 issues for relevant scenarios. Add coverage for the merge behavior described by the issue and run the affected test suite; done means the cases are captured independently of the maxScore bug fix.

Written by the indexing model from the issue text.