Fix rest tests. #14964

Job	Run time
Core Basic (18.x)	7m 3s
Core Content (18.x)	4m 0s
Core Basic (20.x)	6m 34s
Core Content (20.x)	3m 49s
Core Basic - Plone 5 (18.x)	7m 32s
Core Blocks (18.x)	8m 21s
Core Blocks (20.x)	7m 39s
Core Blocks - Listing (18.x)	5m 45s
Core Blocks - Listing (20.x)	5m 35s
Core Control Panels (18.x)	4m 14s
Core Control Panels (20.x)	4m 29s
Coresandbox (18.x)	4m 53s
Multilingual (18.x)	3m 34s
Working Copy (18.x)	5m 10s
Core Volto Slate (18.x)	7m 51s
Project Generator (18.x)	7m 31s
Core Volto Slate (20.x)	7m 30s
Multilingual in Seamless Mode (18.x)	3m 58s
Project Generator (20.x)	7m 2s
Seamless Mode (18.x)	6m 54s
	1h 59m 24s

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