[DPE-3181] fix flakey backup tets #1174

Job	Run time
Unit tests	2m 28s
Lint	13s
Check libraries	37s
Build charms / Get workflow version / Get workflow version	6s
Build charms / Collect charms	10s
Build charms / Build mongodb charm \| base #0	2m 4s
Build charms / Build legacy-application charm \| base #0	1m 16s
Build charms / Build application charm \| base #0	1m 40s
charm-integration	10m 8s
ha-integration	50m 3s
relation-integration	18m 11s
legacy-integration	3m 23s
tls-integration	20m 16s
backup-integration	1h 1m 20s
metric-integration	18m 49s
sharding-integration	2h 0m 16s
sharding-relation-integration	14m 18s
	5h 25m 18s

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