Do not keep unnecessary state across unit tests

Changes:
* Use Where = 'spawn' (default) rather than 'local' (discouraged);
* Use foreach rather than setup.

This change allows each test to assume that the mailbox is clean.
Teach elarm_summary to always use the same ref for notifications...

... also for the first summary.
Merge pull request #29 from lucafavatella/fix-ref-of-first-alarm_summary-notification

Fix ref used in first alarm_summary notification, and improve test