Fixes one liner where the UI was not setting the threat_filters (#89519)

## Summary

Fixes: #89507

* One liner was missed on the front end to persist this
* I fixed some type script issues in the tests which lead to a few missing types in the `DefineStepRuleJson` section
* Adds a unit test for this.

Test instructions:
1. Go to manage detections
2. Create a new rule for Indicator matches
3. Add a filter and click save.
4. Re-load the rule and it should be saved now.

Repeat the steps for edit and it should also be saved there.

You should see this after a reload of the rule after a save for example:

<img width="1038" alt="Screen Shot 2021-01-27 at 4 28 51 PM" src="https://user-images.githubusercontent.com/1151048/106072458-77d1e400-60c5-11eb-8169-fcdadf0f03e7.png">

And this if you have two filters saved in the definition section of the UI:
<img width="1106" alt="Screen Shot 2021-01-27 at 4 30 10 PM" src="https://user-images.githubusercontent.com/1151048/106072539-9a63fd00-60c5-11eb-9c77-bf8224c90016.png">

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios