Tests for issue #6659 - Unvalidated saves and Foreign Keys

This adds tests, one of which fails currently (with descriptive comments of current behaviour) for the behavior of associations.  This is the test code referenced in #6659

The test `test_save_validate_false_after_association_created` is currently failing and I believe that this is a minor Activerecord problem rather than a problem with this test.  When I looked I couldn't find the right place to hook in to offer a fix to the underlying bug but I expect someone more familiar with the framework internals could do so quickly.