Add `testthat` helper state #1740

Job	Run time
R-CMD-check / macOS-latest (release)	5m 49s
R-CMD-check / windows-latest (devel)	1m 29s
R-CMD-check / windows-latest (release)	9m 13s
R-CMD-check / windows-latest (oldrel)	8m 58s
R-CMD-check / windows-latest (4.1)	5m 2s
R-CMD-check / ubuntu-latest (devel)	54m 35s
R-CMD-check / ubuntu-latest (release)	10m 0s
R-CMD-check / ubuntu-latest (4.3)	5m 53s
R-CMD-check / ubuntu-latest (4.2)	9m 21s
R-CMD-check / ubuntu-latest (4.1)	2m 15s
R-CMD-check / ubuntu-latest (4.0)	1m 58s
R-CMD-check / ubuntu-latest (3.6)	2m 14s
	1h 56m 47s

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