slaprunner: fix popen on python3

On python3 several views were not functional, because subprocess output was
retrieved as bytes, but manipulated as a string.

This adds a few tests for views, not complete because it would be too much time
consuming for now.
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3