Use `test`/`_test_utils` to enable single-threaded debug assertions

Use `test`/`_test_utils` to enable single-threaded debug assertions

We have a number of debug assertions which are expected to never
fire when running in a single thread. This is just fine in tests,
and gives us good coverage of our lockorder requirements, but is
not-irregularly surprising to users, who may run with their own
debug assertions in test environments.

Instead, we gate these checks by the `cfg(test)` setting as well as
the `_test_utils` feature, ensuring they run in our own tests, but
not downstream tests.