Bump nunit from 4.1.0 to 4.2.1 in /samples/snippets/core/testing/unit-testing-using-nunit/csharp in the dotnet group across 1 directory

Bump nunit from 4.1.0 to 4.2.1 in /samples/snippets/core/testing/unit-testing-using-nunit/csharp in the dotnet group across 1 directory #1398