This commit adds a test that demonstrates a regression in has_many_through associations
where multiple has_many_through associations associate the same parent with the same
child relation.

The regression is ultimately caused by rails@0d38f2a, which avoids saving a through association target
if we expect the target to be persisted by another (direct) association.

The problem is that if we have multiple has_many_through associations that associate the same parent with the same
child, we will autosave the first set of associations from the parent, but avoid saving the through target a second
time (once it's persisted by the first association), and fail to create the join model for the second association chain.