No error message from test w/o household and SPM info

[nmrodelo]

When working on #315, I discovered that tests missing household and SPM info will run, but if the output they are testing needs to use the size of the household in its calculation, an error message with completely different info is thrown. Not urgent, but good to know, as I imagine we'll run into this again as we add more and more tests.

@nikhilwoodruff

This code:

- name: Family of 3
  period: 2022
  absolute_error_margin: 0.01
  input:
    people:
      person_1:
        age: 30
      person_2:
        age: 50
      person_3:
        age: 35
  output:
    # Ineligible as a family of 3.
    snap_minimum_benefit: 0

Throws this error:

Test 'Family of 3':
    snap_minimum_benefit@2022: [195.84 195.84 195.84] differs from 0.0 with an absolute margin [195.84 195.84 195.84] > 0.01

The proper code looks like this:

- name: Family of 3
  period: 2022
  absolute_error_margin: 0.01
  input:
    people:
      person_1:
        age: 30
      person_2:
        age: 50
      person_3:
        age: 35
    spm_units:
      spm_unit:
        members: [person_1, person_2, person_3]
    households:
      household:
        state_group: CONTIGUOUS_US
  output:
    # Ineligible as a family of 3.
    snap_minimum_benefit: 0

[MaxGhenis]

This is intended behavior, per https://openfisca.org/doc/coding-the-legislation/writing_yaml_tests.html#testing-formulas-by-giving-a-test-case:

Specifying all entities is not mandatory. For any entity that is not specified, all individuals are assumed to be in “a group of their own”. Which means if you do not specify any families, OpenFisca assumes that each individual is the lone member of a one-person family. This applies to Web API payloads as well as test cases.

Since it also applies to the web API, it's unlikely to change, but if something could be clearer you could file an issue to openfisca-core.

[MaxGhenis]

Hmm, one option could be a testing flag that adds each person to the same entity at every level (household/family/spm_unit/tax_unit). @nikhilwoodruff wdyt of this as an openfisca-core request?