test: improve sample_degseq() tests #3052

Job	Run time
Build source R package	6m 34s
Coverage	5m 22s
Check windows-latest (release)	18m 51s
Sanitizer	19m 25s
Check macOS-latest (release)	10m 50s
Check ubuntu-20.04 (release)	6m 38s
Check ubuntu-20.04 (devel)	6m 23s
Check ubuntu-20.04 (oldrel-1)	6m 35s
Check ubuntu-20.04 (oldrel-2)	6m 33s
Check ubuntu-20.04 (oldrel-3)	7m 8s
Check ubuntu-20.04 (oldrel-4)	7m 17s
	1h 41m 36s

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