* Add tests to ensure variant propagation syntax can round-trip to/from string
* Add a regression test for the bug in 35298
* Reconstruct the spec constraints in the worker process
Specs do not preserve any information on propagation of variants
when round-tripping to/from JSON (which we use to pickle), but
preserve it when round-tripping to/from strings.
Therefore, we pass a spec literal to the worker and reconstruct
the Spec objects there.
