The modifications in this commit allows the unit tests
to run on both, pytest 6.x (nose test layout) and the new
pytest 7.x (xunit test layout). This Fixes#2072 in a
much nicer way. Thanks much to @smarlowucf
Use raises as context manager instead of a method decorator.
This clearly identifies which code part is expected to raise
an exception. Related to Issue #1128
Allow to validate the return code from a package manager
operation. In case of zypper the standard UNIX return
code validation does not apply. Return codes from zypper
which are >= 100 are not treated as an error anymore
nose is no longer maintained, thus we have to move to another
testing system. This commit updates the tox setup and all tests
to use pytest instead of nose.
