Test fixes

Something has changed in the behaviour on macOS that triggers failures but unlike the prominent Docker warning about architecture, it's in fact a folder creation issue.

Originally, the tmp_prefix created by beat/beat.core> was used but it looks like it has become inaccessible for some reason. The currently working solution is to create a tmp_prefix in this package rather than use the one from beat/beat.core>. This trigger other issues as it seems that residual stuff were left over. To solve this, adding a clearing of the new folders where appropriate before each test ensures that everything can start fresh.