Reduce unit tests runtime #176
Description
Running the unit tests is taking a long time (circa 10 minutes on CI).
We should try to reduce this.
This is most probably due to running workflows on full loki/sans2d data, which contains many pixels, leading to expensive memory allocations.
We should reduce the data to just a few pixels. We could for example extract a vertical line or slab of pixels, that would still have a discernable scattering pattern, but would be much less expensive.
Note that in the process, we would need a new detector bank shape specific for the tests.