Make get_dir_creation_time use files creation dates, not folder
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We cannot rely on the directory nor file creation date, as it might get modified with filesystem upgrades, as was done recently in January.
File modification date are conserved though, so we must use these when the information is not available from run metadata.
The unit test for it was updated.
@hammerd @ahmedk
@tmichela this is the type of stuff you like