Re-enable some HTG tests previously disabled
!2426 (merged) disables the following vtkHyperTreeGrid tests since they don't work in parallel:
- pvcs.HyperTreeGridDepthLimiter
- pvcs.HyperTreeGridThreshold
- pvcrs.HyperTreeGridDepthLimiter
- pvcrs.HyperTreeGridThreshold
These tests should be enabled again when the vtkHyperTreeGrid is working properly in parallel.