Make the burst_threads test less flaky (#2850)
Make the burst_threads test less flaky (#2850)

Adds synchronization to avoid fewer than one thread per core in the
tool.drcacheoff.burst_threads test, to eliminate flakiness in output
matching.
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio