CommandExecutorService tests improvement

[pierantoniomerlino]

Is your feature request related to a problem? Please describe. The tests implemented for the new CommandExecutorService check the method functionality running simple command on the underlying OS. For example the ls -all command is run and the return code is checked.

This approach is problematic when the tests are run on a OS other than Linux (i.e. Mac OS X). Moreover, the UnprivilegedExecutorService tests are based on the existence of a specific Linux user (i.e. kura), but this cannot be automatically created in automatic build environment.

Describe the solution you'd like The test should be modified to be OS independent.

Describe alternatives you've considered The kura user is created in the BeforeClass section of the tests and it works on Travis, but it not so reliable.

Additional context