The `test_axialExpansionChanger.py` tests are too slow

[john-science]

The unit tests in test_axialExpansionChanger.py are the slowest in ARMI. For instance, these three tests alone take 30 seconds to run:

• TestInputHeightsConsideredHot::test_coldAssemblyExpansion - 15.84 seconds
• TestConservation::test_thermalExpansionContractionConservation_complex - 6.75 seconds
• TestManageCoreMesh::test_manageCoreMesh - 6.08 seconds

Could we reduce the size of the detailed axial expansion test reactor? That is clearly the cause of the slowness:

https://github.com/terrapower/armi/blob/de4eeeec280abf155f19cd709a5f32b9f899fef0/armi/tests/detailedAxialExpansion/refSmallCoreGrid.yaml#L4-L20

Do we really need a full 9-ring reactor to test that assemblies expand thermally, in the axial direction?