Fix uninitialized variable warning on gcc-10.3
gcc 10.3 prints the following warning:
/ccs/home/nthompson/vtk-m/vtkm/worklet/testing/UnitTestBoundingIntervalHierarchy.cxx:54:15: warning: ‘cellId’ may be used uninitialized in this function [-Wmaybe-uninitialized]
54 | return (1 - static_cast<vtkm::IdComponent>(expectedId == cellId));
| ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/ccs/home/nthompson/vtk-m/vtkm/worklet/testing/UnitTestBoundingIntervalHierarchy.cxx:52:14: note: ‘cellId’ was declared here
52 | vtkm::Id cellId;
| ^~~~~~