test: support do-not-merge label #880

Job	Run time
build (3Mi, change-to-secondary)	27m 58s
build (3Mi, incr-backup)	30m 51s
build (3Mi, change-to-standalone)	24m 54s
build (3Mi, backup-failure)	32m 28s
build (10Mi, various-transfer-part-size)	22m 3s
build (3Mi, !//)	36m 54s
build (200Gi, various-transfer-part-size)	22m 52s
build (3Mi, change-to-primary)	29m 44s
build (3Mi, full-backup)	30m 17s
	4h 18m 1s

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