CNDB-15608 port KeyRangeConcatIteratorTest refactoring

[k-rus]

What is the issue

KeyRangeConcatIteratorTest can be improved and is part of Apache patch port in https://github.com/riptano/cndb/issues/15608.

What does this PR fix and why was it fixed

This refactoring improves the test and is isolated from the other patch port, so it make sense to do it separately.

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[k-rus]

CNDB's PR: https://github.com/riptano/cndb/pull/17085

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-2268 rejected by Butler

---

3 regressions found
See build details here

---

Found 3 new test failures

Test	Explanation	Runs	Upstream
o.a.c.index.sai.cql.VectorCompaction100dTest.testZeroOrOneToManyCompaction[ec false]	REGRESSION	🔴⚪	0 / 25
o.a.c.index.sai.cql.VectorSiftSmallTest.testMultiSegmentBuild[ec true false]	REGRESSION	🔴🔴	0 / 25
o.a.c.index.sai.cql.types.collections.maps.MapTimeTest.test[version=eb,dataset=map<time,time>,wide=false,scenario=POST_BUILD_QUERY] (compression)	REGRESSION	🔴🔵	0 / 25

---

Found 5 known test failures