sky: Initialize energest as early as possible

If energest is initialized too late, the radio is not accounted
until the first switch. This is a problem when the radio is always
listening.
Support running multiple random seeds in case one fails
Only fail on real fails
Merge pull request #810 from cmorty/pull/sky_energest

sky: Initialize energest as early as possible / make regression testing more tolerant
framer: Added length function to contikimac-framer.c
framer: Some comments