MINOR: Add integration test for ReassignPartitionsCommand#curReassignmentsToString

[m1a2st]

When I trace code, find that ReassignPartitionsCommand#curReassignmentsToString only have a unit test, but it doesn't have integration test, thus add a integration for it.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)