Add self-testing

[LecrisUT]

It could be possible to just use the same repo to test the plans, but it would not cover different variants. Waiting on feedback from:

• Import tmt plan from the current git repo teemtee/tmt#2339
• Altering imported plans teemtee/tmt#2340

At minimum there needs to be a few plans implemented:

• Plan: trigger_copr_build #4 so that we get artifacts of an example copr project
• Trigger the plans with pre-defined copr build
• Probably packit github actions for interfacing

Considerations:

• Using packit as multi-repo instead of external projects. The issue would be that it becomes ambiguous that the main .packit.yaml is only for testing and not for packaging

[LecrisUT]

Replying to @lachmanfrantisek from #15 (review):

Yeah, I was thinking again lately on how to do that.

• One possibility would be to run tmt itself, but the issue is that these plans are assuming packit environment so that they have /var/share/test-artifacts/*
• Second option would be to have 1 or multiple package projects in the .packit.yaml that we run the tests on. Would be tricky because we would want to store some spec files with known warning, errors, etc.
• Third option would be to call packit commands on arbitrary repos. That one is almost possible with the work on https://github.com/LecrisUT/packit-actions/pulls, the only missing part is calling a packit test as the equivalent of /packit test. I am currently stuck and authentication for that step