Add integration tests for DNF 4 bootc plugin

[evan-goode]

Currently WIP. The plan is to implement a new TMT plan that builds an image for a bootc VM and then runs Behave tests tagged with @bootc inside the VM.

[evan-goode]

Alright, this is in a rough but working state. I created a proof-of-concept test in dnf-behave-tests/dnf/plugins-core/bootc.feature that checks the output of dnf4 bootc status on a freshly-booted VM. Future work will include integrating this with GitHub/Packit, but the test is working locally.

Steps to run the test:

1. Install tmt:

dnf install -y tmt+all

2. Use Tito to build RPMs from the bootc branch of dnf-plugins-core for CentOS Stream 9 and place them in ./rpms

3. Build the bootc qcow2 image (you may need to be root):

sudo ./container-test --container localhost/dnf-bootc:latest bootc_build

The qcow2 will be placed in ./build/qcow2/disk.qcow2 by default.

4. Ensure /var/tmp/tmt is clean, then run the tests with tmt:

sudo rm -rf /var/tmp/tmt
sudo tmt run plans -n bootc-behave-dnf

Full logs (you can search for out: to see Behave output) will be in /var/tmp/tmt/run-00*/log.txt

The whole process is quite slow (existing slowness of building the container image + bootc image building to qcow2 + time to boot/provision the test VM), but that may just be the nature of the beast.