[FEATURE] - Add end-to-end tests

[AlexZzz]

There're no tests right now, except spelling and linter.

Exporter must have a test CI workflow to test most of functionality with real libvirt instances. Not sure about implementation details, but as a result tests must proof that all available metrics are collected and they are collected properly.

It could be useful to check libvirt-exporter data with virsh data when dealing with some metrics.

Might be some issues running libvirt instance during CI.