Add test for packages listing with persistence

[vsedmik]

Problem Statement

SAT-36789 adds the ability to list package persistence and search and sort on that. We should cover that too.

Solution

This PR adds a test case for that and moves the mock helper into a common module-scoped fixture. It also adds the epoch check for the install command (just for consistency and to catch some drift).

Related Issues

https://issues.redhat.com/browse/SAT-36789

PRT test Cases example

trigger: test-robottelo
pytest: tests/foreman/cli/test_imagemode.py -k 'list_packages or containerfile'