Add `assign_core` benchmark to bulk assigner (#2433)
When building benchmark code I found that previous changes to the `build` function in builder only worked for tests and not for benchmarks. The fix I thought of was to have one version of the function for tests which accommodates the mock assigner and another for benchmarks that accommodates whichever assigner a particular runtime is using. There's likely a better solution, so I'd be very open to hearing it! --------- Co-authored-by: antonva <[email protected]> Co-authored-by: command-bot <> Co-authored-by: eskimor <[email protected]>