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!

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Co-authored-by: antonva <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: eskimor <[email protected]>