Fix flaky TestExpendedPostingsCacheIsolation test (#6395) #1892

Job	Run time
build	4m 13s
CodeQL	7m 7s
lint	4m 31s
test	36m 15s
integration-configs-db	2m 42s
integration (requires_docker)	7m 21s
integration (integration_alertmanager)	4m 19s
integration (integration_backward_compatibility)	10m 17s
integration (integration_memberlist)	7m 19s
integration (integration_querier)	15m 6s
integration (integration_ruler)	5m 36s
integration (integration_query_fuzz)	4m 50s
deploy	0s
deploy_website	0s
	1h 49m 36s

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