`test` spec: run as user

[LecrisUT]

I have just encountered an issue where I need the tests to run as non-root user: https://artifacts.dev.testing-farm.io/ee1def5c-2ff2-425c-9c7b-3e915fa7c374/

(I think the issue there is in running mpiexec)

This of course could be very tricky to implement on the tester's side, e.g. making sure the ownership of the files are set accordingly, so it would be nice if this can be handled on the tmt side, particularly maybe having a setting for user: non-root, user: temp, user: (existing user/uid)