Add more tests to Microsoft.Extensions.ServiceDiscovery.Dns and clean up code

[eerhardt]

See PR feedback here:

• Move Microsoft.Extensions.ServiceDiscovery to dotnet/extensions #6868 (comment)
• Move Microsoft.Extensions.ServiceDiscovery to dotnet/extensions #6868 (comment)
• Move Microsoft.Extensions.ServiceDiscovery to dotnet/extensions #6868 (comment)

There doesn't appear to be test coverage for the first 2 methods. So I didn't want to make the suggested code changes without test coverage.

We should ensure this library has adequate test coverage and make the suggested code changes.

[rzikm]

The first item is a fallback on DnsClient.NET which was the previous implementation before it was replaced by managed implementation. The plan is to remove it in couple of releases if the managed implementation is deemed stable, see dotnet/aspire#6104 (comment).

Also, since the code moved, it would be also good to move dotnet/aspire#10311