tumbler: copy files for thumbnailing to a more realistic location

If we have apertis-tests checked out in ~/apertis-tests or something,
it's nice to be able to run the test uninstalled.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3475