So apparently the original upload test fails on missing tests/Keboola/StorageApi/_data/languages.csv.gz - that gets deleted when the compression takes place during async upload test of the non-gzipped file. 
Possible solutions - append a uniqid() to the temporary .gz filename, or create the .gz file in tmp? That would mean initializing the TempService within uploadFile(), but seems like the best solution to me. 
Pro's opinion? :)