ENH: Add test cases for calculation of transfers when receiving Unterhalt or Unterhaltsvorschuss

[ChristianZimpelmann]

When looking at #735, I also checked the test battery and discovered important limitations:

• We have no test for Wohngeld in which the household also receives Unterhalt or Unterhaltsvorschuss -> need to add some
• We have a few test for Kinderzuschlag in which the household receives Unterhalt
  • As expected the Unterhalt is > 0 for the child, not the parent
  • No tests in which the household receives Unterhaltsvorschuss -> need to add some
• We have no test for Unterhaltsvorschuss in which the household receives Unterhalt -> need to add some