1. I don't think we benefit from having two load tests - i.e. one for a single-field file and one for a multi-field file. It'd be fine to just have a test for a multi-field file.
2. We either need a new version of SciTools/iris-test-data with the multi-field file (and the corresponding update to the ".travis.yml" file), or we just use the existing single-field file to construct a multi-field file on the fly (but it would need to tweak to the contents so that it resulted in two distinct Cubes).