Add integration testing loading real files

[SimonHeybrock]

• Load with both scn.load and scn.load_nexus. scippnexus.v2
• Existing files (various facilities).
• Files generated by McStas (+Mantid)
• Can we use Nexus constructor? https://github.com/ess-dmsc/nexus-constructor
• Can this run on GitHub? Or too expensive / too much data?

[SimonHeybrock]

One way to verify results could be to save a "reference" using scipp's HDF5 I/O. If we add more features to the loader we may need to update it, but at least it should catch regressions (e.g., failing to load logs that were previously loaded).

[SimonHeybrock]

Note that test that load files without verification have recently been added under pytest -m externalfile.

Blocked by scipp/scipp#3056.

[SimonHeybrock]

I have begun adding some manual tests in #133. The approach could be extended.

[nvaytet]

Is this now more or less covered by the dmsc integration testing?