add an missing assertion to inheritance_test.rb

assertion for
https://github.com/rails/rails/blob/ad624345e54bd20802de67b2b5c9ef29ecf5
d5f4/activerecord/lib/active_record/inheritance.rb#L32
Merge pull request #9658 from tkhr/add-assert-to-test_company_descends_from_active_record

add an missing assertion to inheritance_test.rb