tmt/testing followups phase 2: split up container-host tests from non-container host

Right now we have a single `integration` plan.

## system-reinstall-bootc

I think we want separate plans for e.g. system-reinstall-bootc which is defined to always come as a standalone binary (package: dpkg/rpm) - and we definitely need a clearly distinct plan for that one.

## Unit tests as integration

We have a lot of unit tests run via `cargo test`. However, there's an interesting pattern of running unit tests even outside of the build as they can actually act as partial integration - what we do is pretty low level so it's certainly possible that e.g. a Linux kernel change breaks our unit tests, and if we run them at integration time we're more likely to find that out.

Basically here we have a container build which captures the binary of `cargo test --no-run` - hmm we should probably generate that as an artifact and push to `ghcr.io/bootc-dev` or so...

## Privileged container tests

There's a lot of our functionality that can be tested as a container, see `just run-container-integration` - this is about making sure we're also running *that* outside of GHA context and especially bridging to the tmt world. This one again has nothing to do with whether the host is a bootc system.