Add test coverage for enableChildChains.

(cherry picked from commit d352b5ec)

Bug: 37641280
Test: marlin builds and boots
Test: new unit test passes
Test: netd_{unit,integration}_test pass
Change-Id: Ie39a6eac1707e4a1b085a02c2c7b1976806f4c0c
Merged-In: I6fb056abeb45ab1fc2bc45bcf2d8f9a95114b1d9