fix random string used in test allocating 50MB in Cortex binary #8827

Job	Run time
test	18m 16s
test	18m 16s
lint	3m 46s
lint	3m 46s
build	3m 57s
build	3m 57s
CodeQL	7m 59s
CodeQL	7m 59s
integration (requires_docker)	5m 23s
integration (requires_docker)	5m 23s
integration (integration_alertmanager)	3m 56s
integration (integration_alertmanager)	3m 56s
integration-configs-db	2m 3s
integration-configs-db	2m 3s
integration (integration_backward_compatibility)	8m 13s
integration (integration_backward_compatibility)	8m 13s
integration (integration_memberlist)	7m 38s
integration (integration_memberlist)	7m 38s
integration (integration_querier)	11m 48s
integration (integration_querier)	11m 48s
integration (integration_ruler)	4m 12s
integration (integration_ruler)	4m 25s
integration (integration_query_fuzz)	2m 28s
integration (integration_query_fuzz)	2m 28s
deploy	0s
deploy	0s
deploy_website	0s
deploy_website	0s
	2h 39m 31s

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