Integration testing package

[SimonHeybrock]

We and ECDC need to run integration testing between ESS NeXus files and the Scipp ESS packages. This will need to be run in multiple places such as the JSON template file repo as well as CODA. We therefore would like to implement test helpers in a centralized place.

I think the helpers will ultimately depend on the ess<technique> packages, so having this in essreduce may not be ideal. We should consider creating ess_integration_testing or something similar.

Incomplete list of what the helpers may need to to:

• Run Chexus.
• Load with ScippNexus, raise if there are unexpected warnings.
• Load with GenericNeXusWorkflow. This will require knowledge of the expected or required monitor and detector names.
• Load with instrument-specific loader.