Add steps/network tests

[yitsushi]

What this PR does / why we need it:

Testing network interface steps, because it had zero tests and we are before a not so small refactoring, more tests we have, more likely we can catch an error.

While I was working on #144, I found a strange code part and wrote it down as note to check back because it's not a bug, and wanted to verify it shouldn't be executed in any cases, at least my intuition told me that. The easiest way to check, writing test and yes, in out current plan model, it's a dead code.

Which issue(s) this PR fixes:

Related to #155

Special notes for your reviewer:

Added testing for a case that can never happen based on our code, but the intention seems like we want to call it, it just not on the right place or something.

In core/steps/network/interface_create.go, in the Do function, we have this block:

exists, err := s.svc.IfaceExists(ctx, deviceName)
if err != nil {
	return nil, fmt.Errorf("checking if networking interface exists: %w", err)
}
if exists {
	details, err := s.svc.IfaceDetails(ctx, deviceName)
	if err != nil {
		return nil, fmt.Errorf("getting interface details: %w", err)
	}

	s.status.HostDeviceName = deviceName
	s.status.Index = details.Index
	s.status.MACAddress = details.MAC

	return nil, nil
}

The ShouldDo returns false if the device is there, so that if exists { ... } in the Do function will never be executed.

Checklist:

• squashed commits into logical changes
• adds unit tests

[Callisto13]

really clear and simple 🎉
just a couple of notes

[Callisto13]

lgtm

before you merge, can you make all the common string values variables? for easy changing in future

[yitsushi]

@Callisto13: Moved all inline strings into constants and added a helper
function to create a new "full" interface. That was repeated multiple times,
now they are just a fullNetworkInterface() call.

Edit: Aaaand you approved before I could mention you and ask for a new
stamp. 😆

[Callisto13]

I happened to be lurking nearby 👀