`onlineddl_scheduler` test: fix flakiness in artifact cleanup test (#… #498

Job	Run time
Build and push vitess/lite Docker images (latest)	3m 4s
Build and push vitess/lite Docker images (mysql57)	2m 55s
Build and push vitess/lite Docker images (mysql80)	2m 57s
Build and push vitess/lite Docker images (percona57)	2m 58s
Build and push vitess/lite Docker images (percona80)	3m 13s
	15m 7s

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