support/testing: add sudo package test

Create a new user 'sudotest' to validate that sudo really works (i.e.
properly has setuid).

Creating the user and adding it to sudoers is done at runtime, otherwise
we'd need to add extra files to the config which complicates things a
little bit.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>