Add wait commands to block-listing tests #12810

Job	Run time
Core Basic (18.x)	6m 40s
Core Basic (20.x)	7m 7s
Core Blocks (18.x)	7m 4s
Core Basic - Plone 5 (18.x)	7m 26s
Core Blocks (20.x)	7m 51s
Core Blocks - Listing (18.x)	6m 28s
Core Blocks - Listing (20.x)	5m 43s
Core Control Panels (18.x)	3m 59s
Core Control Panels (20.x)	4m 20s
Core Content (18.x)	3m 16s
Core Volto Slate (18.x)	7m 18s
Core Content (20.x)	3m 37s
Core Volto Slate (20.x)	7m 20s
Project Generator (18.x)	5m 57s
Coresandbox (18.x)	4m 13s
Project Generator (20.x)	6m 25s
Multilingual (18.x)	4m 6s
Multilingual in Seamless Mode (18.x)	3m 38s
Working Copy (18.x)	3m 35s
Seamless Mode (18.x)	6m 58s
	1h 53m 1s

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