Updates to dotnet example automation for recent changes on dotnet side

[ArcturusZhang]

In azure-sdk-for-net, we are changing how the namespace works.
Previously, to avoid NUnit related namespaces to be added into the using statements, we write code in this way:

[NUnit.Framework.Test]
public async Task Sample_DoSomething() {}

Because of a refactor of code in dotnet generator, the above way is no longer possible to write - therefore the new code looks like this:

using NUnit.Framework;

[Test]
public async Task Sample_DoSomething() {}

Therefore this PR is changing how we collect those namespaces in usings to exclude those NUnit namespaces.

Unit test cases are also updated.

Related .Net generator changes here:

• change the attributes in generated cases autorest.csharp#5150
• Refactor sample generator in mgmt autorest.csharp#5107