Fix runner client

[stan-is-hate]

Deflake implementation moved try-except-finally check inside the loop - so if an exception happens when reporting another exception (inside except BaseException), inner finally block inside the loop will execute, but everything outside the loop won't - meaning runner_client will never report the results back. This is a rare case, and it's unlikely to come up in real world, but it is possible. Fixed by extracting a single test execution into a separate function, and wrapping a deflake loop in a try-finally. No need for except, we'll let the inner exception from the _do_run to bump through. Also updated the unit test to how it was before deflake.

[CLAassistant]

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