Bump NUnit from 4.1.0 to 4.2.1 in /docs/core/testing/snippets/order-unit-tests/csharp in the dotnet group across 1 directory

Bump NUnit from 4.1.0 to 4.2.1 in /docs/core/testing/snippets/order-unit-tests/csharp in the dotnet group across 1 directory #1779