Fix flaky TestExpendedPostingsCacheIsolation test #10599

Job	Run time
test	15m 53s
test	15m 42s
build	4m 16s
build	4m 16s
CodeQL	7m 3s
CodeQL	7m 3s
lint	4m 21s
lint	4m 21s
integration-configs-db	3m 4s
integration-configs-db	3m 4s
integration (requires_docker)	7m 14s
integration (requires_docker)	7m 14s
integration (integration_alertmanager)	4m 11s
integration (integration_alertmanager)	4m 11s
integration (integration_backward_compatibility)	11m 0s
integration (integration_backward_compatibility)	11m 0s
integration (integration_memberlist)	7m 30s
integration (integration_memberlist)	7m 30s
integration (integration_querier)	14m 51s
integration (integration_querier)	14m 51s
integration (integration_ruler)	5m 49s
integration (integration_ruler)	5m 49s
integration (integration_query_fuzz)	4m 52s
integration (integration_query_fuzz)	4m 52s
deploy_website	0s
deploy_website	0s
deploy	0s
deploy	0s
	2h 59m 57s

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