Create some unit testing for Mdns::StartServer

[bzbarsky-apple]

Problem

We don't really have any tests for Mdns::StartServer. This leads to major bugs like #7911 not being noticed, and workarounds being put in place (like the AdvertiseOperational call instead of StartServer in emberAfOperationalCredentialsClusterAddNOCCallback

Proposed Solution

Add unit testing for this code.

@cecille @andy31415

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.