Run ActiveModel test suites in random order.

This gets the whole ActiveModel test suites working even if
`self.i_suck_and_my_tests_are_order_dependent!` is disabled
in `ActiveSupport::TestCase`.

Two places are found that potentially leak global state. This patch
makes sure states are restored so that none of the changes happen in
a single test will be carried over to subsequence tests.
Merge pull request #14315 from zuhao/activemodel_tests_in_random_order

Run ActiveModel test suites in random order.