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.
Enhance readability of ActionView DateHelper#distance_of_time_in_words

Refactor numerical constants to module constants which give the numbers
a contextual meaning. This commit aims to provide quicker understanding
for part of the implementation of the
DateHelper#distance_of_time_in_words method.
Merge pull request #14334 from kennym/master

Enhance readability of ActionView DateHelper#distance_of_time_in_words
Cleanup Parrot after each test.
Merge pull request #14332 from tgxworld/autosave_association_test

Delete instances of Parrot after each test.