[Fact]
public void TestSubject()
{
  // Arrange
  string expected = "Some random subject";
  InsertMessage(m => m.Subject = expected);

  // Act
  // ...

  // Assert
  // ...
}

private void InsertMessage(Action<Message> alterMessage)
{
  using (MessageContext fakeContext = GetFakeContext())
  {
    var message = new Message("ignored id", content: new byte[0]);
    alterMessage(message);

    fakeContext.Insert(message);
    fakeContext.SaveChanges();
  }
}