Improving testing data management for "self-test"

[lassoan (Andras Lasso)]

@jcfr

using file from the cache, they will be copied into a temporary location. This applies to SampleData download

We should be able to load data directly from cache. We don’t even know what files we need to copy to a temporary location (we often need multiple files to be able to load data, e.g., for nhdr+raw files, MRML scenes, DICOM data).