public class TransposeRandom {

  [Test]
  public void Test1() {
    string message = "PAmuapYIR cBANYpCm lETQoY";
    string output = TransposeEncryption.Encrypt(message, 4); // True
    Assert.AreEqual("parnmq apcylo mybpey uiact", output);
  }

Nope, this is not random test, it's still fixed test.

Random does not mean

getRandom()
{
    return 4 // chosen by fair dice
}

It means you need to generate random cases on the fly.