Improve testing of charm libraries

[NucciTheBoss]

Per our discussion on the dnf charm library pull request here #75 (comment), we realized that there is potentially a better way to launch tests without having to use pytest's built-in flag --ignore=... to ignore specific tests. In the discussion thread, it was suggested to use @pytest.mark.<test_suite> with pytest -m <test_suite>.

I also wanted to suggest potentially using cleantest for helping with the overhaul of the tests for the charm libraries. I noticed in the tox.ini file that there is a standalone integration environment for just setting up an LXD test instance:

operator-libs-linux/tox.ini

Lines 70 to 88 in d0ebc9f

	[testenv:integration]
	description = Build a LXD container for integration tests then run the tests
	allowlist_externals =
	lxc
	bash
	commands =
	# Create a LXC container with the relevant packages installed
	bash -c 'lxc launch -qe ubuntu:focal {[vars]lxd_name} -c=user.user-data="$(<{[vars]itst_dir}/test_setup.yaml)"'
	# Wait for the cloud-init process to finish
	lxc exec {[vars]lxd_name} -- bash -c "cloud-init status -w >/dev/null 2>&1"
	# Copy all the files needed for integration testing
	lxc file push -qp {toxinidir}/tox.ini {[vars]lxd_name}/{[vars]lxd_name}/
	lxc file push -qp {toxinidir}/pyproject.toml {[vars]lxd_name}/{[vars]lxd_name}/
	lxc file push -qpr {toxinidir}/lib {[vars]lxd_name}/{[vars]lxd_name}/
	lxc file push -qpr {[vars]tst_dir} {[vars]lxd_name}/{[vars]lxd_name}/
	# Run the tests
	lxc exec {[vars]lxd_name} -- tox -c /{[vars]lxd_name}/tox.ini -e integration-tests {posargs}
	commands_post =
	-lxc stop {[vars]lxd_name}

cleantest can handle both bringing up and tearing down the Ubuntu test instance. This way the LXD logic does not have to live inside of tox.ini and the integration environment can be dedicated for just launching the integration tests.