test: add DNS tests without interface configuration

[mkowalski]

This PR adds a set of tests where we configure DNS servers without providing any per-interface configuration. This is a canonical way to use k-nmstate to provide global DNS configuration. It is useful in environments where we are not allowed to modify network interfaces, but we still want to reconfigure DNS.

Is this a BUG FIX or a FEATURE ?:

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/kind bug /kind enhancement

What this PR does / why we need it:

Special notes for your reviewer:

Release note:

NONE

[mkowalski]

/test pull-kubernetes-nmstate-e2e-operator-k8s

[mkowalski]

/test pull-kubernetes-nmstate-unit-test

[qinqon]

/lgtm /approve

[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qinqon

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[mkowalski]

/test pull-kubernetes-nmstate-unit-test

[mkowalski]

/retest