title: NSubstitute mock extension method
body: <p>I want to do mock extension method, but it does not work. How can this be done? </p>
  
  <pre><code>public static class RandomExtensions
  {
      public static IEnumerable&lt;int&gt; NextInt32s(this System.Random random, int neededValuesNumber, int minInclusive, int maxExclusive)
      {
          // ...
      }
  }
  </code></pre>
  
  <hr>
  
  <pre><code>[Fact]
  public void Select()
  {
      var randomizer = Substitute.For&lt;DefaultRandom&gt;();
      randomizer.NextInt32s(3, 1, 10).Returns(new int[] { 1, 2, 3 });
  }
  </code></pre>