Verify SSI computation for an ineligible parent with an eligible child

Today I demoed our SSI computation to a team of disability policy experts, and one expected a lower SSI benefit in this case:
* Single parent of one blind child
* $2,000 monthly employment income
* **We estimate $744.50 monthly SSI**

My quick pass at what I understand the computation to be is:
* $841 base amount
* Parent's countable earned income pre-percent-earnings-exclusion will be $2,000 - $20 general exclusion - $65 earned income exclusion = $1,915
* Parent's countable earned income is $1,915 / 2 = $957.50
* Deemed income is parent's countable income minus parental allocation: $957.50 - $841 = $116.50
* Benefit is $841 - $116.50 = $724.50

Per #1302, that's pretty much what's in the [computation tree](https://gist.github.com/MaxGhenis/377c736e54a6202075d44fdc1d2d51dd), but it lands at $744.50, so I'm missing a $20 somewhere.

Let's add this as an integration test case and check the logic.