Test for indenting list item more than one level

[flarnie]

After we merge #762 then Draft should support indentation of a list more than one item deep.

To make sure we don't regress this support in the future, let's add a test.

I would add a test in the suite of unit tests for RichEditorUtil, here ->
https://github.com/facebook/draft-js/blob/master/src/model/modifier/__tests__/RichTextEditorUtil-test.js

We don't seem to have any test for 'onTab' so let's add one.

[simsim0709]

@flarnie. Can I try this issue?

[flarnie]

Go for it @simsim0709 :)

[simsim0709]

@flarnie ,

This is first time to make a PR on draft-js, so it might be wrong.
I need some instruction if it's wrong.

I have a question: Do I need to write other test case other than this(depth check)?

Thank you.