Fix test metrics hierarchies

[mastastny]

Error was caused by deletion of custom_ui_backend while it was still registered in custom_ui_product.
This fix assures, that teardown of custom_ui_product fixtures will be executed before teardown of custom_ui_backend.

[dhlavac]

Adding dependency for this teardown is kind of no go, I would suggest using one of below approaches:

• change finalizer - decorate it with backoff, create custom finalizer
• create custom_ui_service that binds by default product and backend but is parametrized and in case of this test it will be created without bind, but finalizer will handle that
• extend the test with not only adding but also unassigning backend from product