Check the count of the families in a default collection.

After I9e01d237c9adcb05e200932401cb1a4780049f86, Minikin can
accept up to 254 font families. To make sure we don't exeed this limit,
introduce an assertion to fontchain_lint.py

Bug: 33562608
Test: m fontchain_lint
Change-Id: I35fc104fd4763f77145bef0e7c5facf30fa99644