private PartitionedRegion getPartitionedRegion(String regionName) {
    return (PartitionedRegion) getCache().getRegion(regionName);
  }
Regression test for TRAC #39868

<p>
TRAC #39868: PartitionMemberDetails.getSize() reports negative PR sizes when redundancy is 0
geode-core/src/distributedTest/java/org/apache/geode/internal/cache/PartitionedRegionSizeDUnitTest.java