Improve test data download script to always remove untarred folder
We had a weird hick-up of the test suite in tonight's scheduled CI run (https://gitlab.cern.ch/corryvreckan/corryvreckan/-/jobs/5517155). The only explanation I see is that there was some outdated data laying around.
This MR adapts the test data download script to remove the untarred folder (which is not checksummed) and untars the data always from (possibly buffered) file.