common: smartd.conf: disable long self-tests

The disks on gemini show increased write latency every now and then, and this appears to correlate with when long self-tests are running.

Fixes #498 (closed)

requested review from @jelle

#498 (closed) aside, not having the disks do self-tests for half week every week seems like a good idea. ️

approved this merge request

mentioned in commit 230571f2

merged