Preconditions in Swift and Testing Them

In Java I can use frameworks like Guava to check method preconditions, for example like this:

public void doSomething(int index) {
  Preconditions.checkArgument(index >= 3);
  // do something
}

And I can then test them like this:

@Test(expected = IllegalArgumentException.class)
public void testDoSomething() {
  sut.doSomething(2);
}

How can I do the same thing in Swift?

I know about Swift's assertions, but I don't know how you would test for them.