[DPP-76] Intermittent test failures

[qnikst]

[Imported from JIRA. Reported by Tim Watson [Administrator] @hyperthunk) as DPP-76 on 2013-06-10 20:12:16]
The supervisor tests seem to fail intermittently. This is almost certainly because the tests rely on ordering guarantees that the CH doesn't really offer - viz ordering between multiple producers and 1 test case process, or ordering of monitor notifications - yet we should try to do something about this.

It seems easier to provoke failures by running with {{+RTS -N1}}