[6.16.z] better logging of fam failures

[Satellite-QE]

Cherrypick of PR: #16967

Problem Statement

The output of FAM tests is mangled due to how pytest tries to print the big result object.
There are no linebreaks and it's super hard to find the failure.

Additionally the double assert is pointless, as it asserts twice the same thing: the result is successful.

Solution

Stop asserting PASSED in stdout (additionally saves the case where there is the string PASSED in the output for some reason, but the test actually failed).
Pass a custom error output to the status == 0 assertion, including stdout, stderr and the status code.

Related Issues

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/sys/test_fam.py::test_positive_import_run_roles

[Satellite-QE]

PRT Result

Build Number: 9463
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/sys/test_fam.py::test_positive_import_run_roles --external-logging
Test Result : ================== 1 passed, 9 warnings in 560.04s (0:09:20) ===================