Instead of hard-coding the use of the "org.osbuild.linux" runner,
use the new `osbuild.pipeline.detect_host_runner` function to
dynamically detect the runner for the host system. That should fix
the tests on RHEL systems, where python3 is by default not present
and even if it is manually installed, is an indirection via
alternatives (i.e. a link to /etc/alternatives), which must be
explicitly configured in the build root container for the host.
Change the default of libdir to /usr/lib/osbuild and
remove redundant logic. Additionally, change how the
python package is detected.
Instead of checking if libdir is None, check if
/usr/lib/osbuild is empty - i.e. if the user has specified
a different directory than the default.
Create small test cases that check the execution of Stages and
Assembler. This ensure that path handling, the sandbox, as well
as basic result reporting works as expected.
Move the `test_osbuild.py` test into the module-test directory. This
test contains just a bunch of basic functionality tests for a selection
of osbuild modules. Hence, it can be run together with the other module
tests.
