build: darwin: disable -fstrict-aliasing warnings

gcc 4.2.1 as shipped with Xcode complains incessantly about aliasing
warnings, which, while technically true, disregards the fact that the
aliased types have the same layout in memory. Squelch the warnings.
test: fix signed/unsigned compiler warning
test: add 'start timer from check handle' test

Check that a timer that is started from a check handle gets picked up
correctly, i.e. that it influences the timeout used in the next tick
of the event loop.