Improve tests/setup.sh

Snot's exit code is now carried over from run to fin.
Make snot a dependency of test target in Makefile
Improved timing

Notifications are now printed at least once and as often as they are
supposed to otherwise.

Expiration now starts exactly when the first print of a notification
happens.

A delay of zero is handled gracefully.

Prints may happen up to 200msec before they are supposed to, so being
slightly early doesn't set you back a complete cycle. This number may
need tweaking.