Guard VTXWriter use when ADIOS2 is unavailable

[masaqif]

Skip the ADIOS2 VTXWriter test module when DOLFINx itself is built without ADIOS2 support, while still using the Python adios2 import check for tests that inspect BP output.\n\nAlso guard VTXWriter output in the scattering boundary conditions demo so the demo can continue when ADIOS2-backed VTX output is unavailable.\n\nFixes #3995.\n\nSplit out from #4249 per maintainer request.

[francesco-ballarin]

While you are at it, you may also want to look at the following other places which show a similar issue. This is how we patch it for debian, but there are for sure other ways to handle that (e.g., as you did, moving the adios2 import or skip earlier, when that is used inside the test)

https://salsa.debian.org/science-team/fenics/fenics-dolfinx/-/blob/25530d9953e688fa1848ea159c1469f7f89bc3b7/debian/patches/test_adios2_skip.patch
https://salsa.debian.org/science-team/fenics/fenics-dolfinx/-/blob/11ba5c5df4435136ca793d6b63911f4543e82552/debian/patches/demo_no_vtk_adios2.patch

[masaqif]

Thanks for the pointers. I expanded this beyond the original import-order fix: the ADIOS2 test module now skips when DOLFINx is built without ADIOS2, and the scattering boundary conditions demo guards its VTXWriter output similarly.

[schnellerhase]

Can you link #3995 so it auto-closes on merge?

[drew-parsons]

Looks good to me too, will help avoiding having to dance around different configurations for the 32-bit architectures.