prog: make TestRotationCoverage faster #14889

Job	Run time
dashboard	3m 20s
aux	5m 5s
build	11m 9s
arch (presubmit_arch_linux)	10m 51s
gvisor	4m 16s
arch (presubmit_arch_freebsd)	6m 41s
old	6m 41s
race	8m 40s
arch (presubmit_arch_netbsd)	4m 26s
arch (presubmit_arch_openbsd)	4m 25s
race_dashboard	5m 0s
arch (presubmit_arch_darwin)	4m 23s
arch (presubmit_arch_windows)	3m 24s
arch (presubmit_arch_executor)	3m 16s
	1h 21m 37s

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