pkg/podman: Image interface Unit tests

Provides Go Unit tests for the Image interface implementation
introduced in commit fd715a9. Tested were results of Podman
starting on version 1.1.2, which should be sufficient since
the minimum required Podman version is 1.6.4 (see commit
8e80dd5). Covered are structures representing podman commands
'podman inspect --type image' and 'podman images'.

Signed-off-by: Dalibor Kricka <dalidalk@seznam.cz>

Author: DaliborKr