Add AM test for after_validation returning false

This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358

It's simply a clarification of the current behavior by which if an
`after_validation` ActiveModel callback returns +false+, then further
`after_` callbacks **are not halted**.
Add AM test: after/around callback returning false

This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358

It's simply a clarification of the current behavior by which if an
`after_` or `around_` ActiveModel callback returns +false+, then the callback
chain **is not halted**.

The callback chain in ActiveModel is only halted when a `before_`
callback returns `false`.
Merge pull request #18032 from claudiob/add-test-for-after-validate-callbacks

Add test for ActiveModel `after_validation`, `after_` and `around_` callbacks returning false