add unit test coverage for RegionPlan

[aglinxinyuan]

Task Summary

Add RegionPlanSpec covering the lookup and DAG-iteration contract of RegionPlan:

• getRegion(id) returns the region with the given id; throws NoSuchElementException for unknown ids
• getRegionOfLink returns the region whose physicalLinks contain the link; throws NoSuchElementException when no region claims it
• getRegionOfPortId returns the region whose ports contain the global port id, or None when no region claims it
• topologicalIterator yields region ids in topological order based on regionLinks

Priority

P3 – Low

Task Type

• Testing / QA