test_server: Allow init_per_ function to throw skip

Allow all init_per_[suite|group|testcase] to throw (or exit)
{skip, Reason} to explicitly ignore tests without failing.

It is easier to catch it in one place instead of writing a try
in each init_per_* to return {skip, Reason}.
Merge branch 'dgud/test_server/throw_skip'

* dgud/test_server/throw_skip:
  test_server: Allow init_per_ function to throw skip