Add support for running commands as an ordinary user

[glum23]

It'd be very useful to run some commands in the tests also as a non-root user, e.g.
Given user "user" (plus possibly some detailed specification like group membership)
When I run "cmd" as user "user"
...

[ignatenkobrain]

When I run "cmd" as "user" would be best.

[kkaarreell]

I would say we should have an extra step also for the user account creation.