private static SystemStreamMetadata buildSystemStreamMetadata(int numPartitions) {
    SystemStreamMetadata systemStreamMetadata = mock(SystemStreamMetadata.class);
    Map<Partition, SystemStreamMetadata.SystemStreamPartitionMetadata> partitionToMetadata =
        IntStream.range(0, numPartitions)
            .boxed()
            .collect(
                Collectors.toMap(Partition::new, i -> mock(SystemStreamMetadata.SystemStreamPartitionMetadata.class)));
    when(systemStreamMetadata.getSystemStreamPartitionMetadata()).thenReturn(partitionToMetadata);
    return systemStreamMetadata;
  }
Unfortunately, we still need powermock for testing the regex topic flow, since that flow checks for a specific
rewriter class, and setting up the support for that specific class is too cumbersome.
Many of the tests do not need powermock.
samza-core/src/test/java/org/apache/samza/coordinator/TestJobModelCalculator.java