add command line flag to collect_steps.py for multi-threading (#966) #226

Job	Run time
Measure coverage	8m 49s
Additional Builds and Concurrency Tests	7m 14s
miri	3m 54s
Basic Checks	7m 8s
Release builds and tests	6m 21s
sanitize (address)	4m 24s
sanitize (address, multi-threading)	5m 53s
sanitize (leak)	3m 31s
sanitize (leak, multi-threading)	3m 15s
	50m 29s

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