> Can you run tests serially in the (horrible) case when tests need to build on one another?

Yes: https://nexte.st/docs/configuration/test-groups/ (edit: though tests that build on each other is a bit harder — test groups are meant for when tests need access to a shared resource)

> How are you querying for the tests? Is that just built into rust's test stuff?

Just running --list against test binaries.

> Would it be possible to fork the test process? It'd be pretty interesting if you could spawn a test process, and then fork it for each test to save both on memory and any static state stored within the test.

This is possible in principle, but nextest doesn't really inject itself into tests like that (injection can cause reliability issues in practice, and a big focus of nextest is reliability). Forking is also not possible in multithreaded programs.