sky: Initialize energest as early as possible / make regression testing more tolerant

This is an updated version of #670 (github does not let me reopen it).

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.
Unfortunately the changes in timing cause the regression test to fail. I therefore also extended the regression testing to give each test a second chance.
NB:  @adamdunkels , @fros4943 : I can't reproduce the issue on my local machine, even when using the compiler used for travis. As far as I can see Java's PRNG is the most likely reason for that. It might be a good Idea to replace the library-PRNG with a deterministic one.
NB: Should I put the changes in the test system into an extra PR?