Add assertIsIndeterminate for Ultron Compose

[Darocs]

I think you should also add a CheckBox check for Indeterminate. Not only assertIsOn/assertIsOff, but also assertIsIndeterminate. Because I have a CheckBox, with three states: true, false, null. When null, the ToggleableState is Indeterminate. And I would like to check this.

You can see how I solved this Issue in my Validation_Etude repository at this path: app/src/androidTest/java/en/polescanner/validation_etude/ui/reusable/components/Assertions.kt

If this idea doesn't work for you, answer this Issue so I don't expect a positive outcome.