test(e2e): delete MantleBackups after backup-failure tests to avoid interfering with subsequent tests #1326

Job	Run time
build (1.35.4, 3Mi, backup-failure)	1h 20m 43s
build (1.35.4, 3Mi, change-to-secondary)	30m 29s
build (1.35.4, 3Mi, !//)	46m 34s
build (1.35.4, 3Mi, change-to-primary)	33m 5s
build (1.35.4, 3Mi, incr-backup)	38m 39s
build (1.35.4, 200Gi, various-transfer-part-size)	23m 0s
build (1.35.4, 3Mi, lock)	23m 40s
build (1.35.4, 3Mi, full-backup)	34m 48s
build (1.35.4, 3Mi, change-to-standalone)	25m 2s
build (1.35.4, 10Mi, various-transfer-part-size)	22m 35s
build (1.34.4, 3Mi, full-backup)	36m 9s
build (1.35.4, 3Mi, change-to-primary)	33m 5s
build (1.35.4, 3Mi, change-to-secondary)	30m 29s
build (1.35.4, 3Mi, incr-backup)	38m 39s
build (1.35.4, 3Mi, change-to-standalone)	25m 2s
build (1.35.4, 3Mi, backup-failure)	1h 20m 55s
build (1.35.4, 3Mi, !//)	46m 34s
build (1.35.4, 3Mi, lock)	23m 40s
build (1.34.4, 3Mi, full-backup)	36m 9s
build (1.35.4, 10Mi, various-transfer-part-size)	22m 35s
build (1.35.4, 200Gi, various-transfer-part-size)	23m 0s
build (1.35.4, 3Mi, full-backup)	34m 48s
	13h 9m 40s

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