Expose a custom view filter based on custom field in another node
Thanks to the clues provided by Geoff in a comment, I found the answer.
My mistake, (doing too much work), was  to create my own foreign key, test_set_id.   Drupal uses node_reference fields for such joins. Using node reference, associated content can be directly accessed after adding the relationship in the set-up view screen, without any code.
I also ended up creating an additional master parent content type, to hold some generic test_set information, and to give the child nodes something to point to.