Make `later_fd()` tests more robust (#202) #266

Job	Run time
R-CMD-check / setup	8s
routine / routine	2m 13s
website / pkgdown	1m 35s
R-CMD-check / macOS-latest (4.4.2)	1m 59s
R-CMD-check / windows-latest (devel)	5m 4s
R-CMD-check / windows-latest (4.4.2)	4m 48s
R-CMD-check / windows-latest (4.3)	9m 26s
R-CMD-check / windows-latest (4.2)	7m 18s
R-CMD-check / windows-latest (4.1)	5m 43s
R-CMD-check / windows-latest (3.6)	5m 56s
R-CMD-check / ubuntu-latest (devel)	2m 31s
R-CMD-check / ubuntu-latest (4.4.2)	2m 21s
R-CMD-check / ubuntu-latest (4.3.3)	2m 31s
R-CMD-check / ubuntu-latest (4.2.3)	2m 29s
R-CMD-check / ubuntu-latest (4.1.3)	2m 57s
R-CMD-check / ubuntu-latest (4.0.5)	2m 34s
	59m 33s

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