Testing: Write loom tests for scheduling and add to CI

[bjchambers]

Summary
Loom let's us test code using atomics (counters, mutexes, etc.) in an "all interleavings manner". It requires compiling the code in a different configuration, and updating the imports to use only loom primitives when that configuration is present.

We should add some more loom tests for the scheduler, and see if we can get at least that crate (and other partitioned pipelines) running using it in CI.

This would provide more confidence in the details of #409.