This brings in the following changes:

- nexedi/nxdtest@1e6a1cc6
- nexedi/nxdtest@b5a74214

The first one makes nxdtest more robust to detect an error in its worker
thread, while the second one makes nxdtest more human-friendly when used
while developing.

/reviewed-by @jerome
/reviewed-on nexedi/slapos!918