How to test libdnf5 cpp plugins

[kontura]

There are a couple of tests that create a Python dnf4 plugin directly in the test environment to perform certain tests:

• https://github.com/rpm-software-management/ci-dnf-stack/blob/main/dnf-behave-tests/dnf/plugin-api.feature
• https://github.com/rpm-software-management/ci-dnf-stack/blob/main/dnf-behave-tests/dnf/plugins-removal.feature (which uses plugins defined in specs: https://github.com/rpm-software-management/ci-dnf-stack/tree/main/dnf-behave-tests/fixtures/specs/plugins-callbacks)

libdnf5 cpp plugins need to be compiled, do we want to create additional COPR repository with test plugins?
Possibly some tests could remain in python and use the python_plugins_loader plugin, however it doesn't seem to work currently, reported as rpm-software-management/dnf5#1775.