Bump NUnit3TestAdapter from 4.5.0 to 4.6.0 in /samples/snippets/core/testing/unit-testing-using-nunit/csharp in the dotnet group

Bump NUnit3TestAdapter from 4.5.0 to 4.6.0 in /samples/snippets/core/testing/unit-testing-using-nunit/csharp in the dotnet group #1207