Expand live simulation integration coverage across US and UK

In #2384, we're looking to remove the Oregon rebate tests, which merely run the simulation over a particular policy. These tests are slow and can be clunky, so I think we should proceed with #2384, but they do serve as a basic integration/smoke test that ensures simulations run properly and they did successfully catch issues in #2398.

We should create one or more simulation integration tests. What I'd propose is more than 1, but fewer than 6, targeted at certain very important key policies. The list should include the following policies:

1. The US Earned Income Tax Credit and/or Child Tax Credit
2. The UK Universal Credit
3. A bracketized policy (e.g., income tax)
4. A state policy in a state with a high level of overall tax complexity (I'd nominate some program in California)

And should simultaneously deal with the following configurations:

1. A standard US run
2. A standard UK run
3. A US state run
4. A UK national run (i.e., England, Wales, Scotland, or Northern Ireland)

I do think we should remove the Oregon tests, but wait to merge #2388 until we can add at least one more relevant integration test.

Start by reading #2384, #2388, and #2398, then locate the existing Oregon simulation tests and their integration-test setup. Define one or more tests covering the listed US and UK policies and run configurations, while preserving basic simulation smoke coverage. Done means the new tests pass and provide coverage before the Oregon tests are removed through #2388.