kv/etcd: Fix race condition within unit test for Watch method (#6479) #1955

Job	Run time
lint	4m 20s
test	15m 55s
test-no-race	7m 23s
build	4m 5s
CodeQL	6m 46s
integration-configs-db	3m 3s
integration (requires_docker)	7m 19s
integration (integration_alertmanager)	4m 27s
integration (integration_backward_compatibility)	10m 31s
integration (integration_memberlist)	7m 29s
integration (integration_querier)	14m 57s
integration (integration_ruler)	5m 56s
integration (integration_query_fuzz)	4m 40s
deploy_website	0s
deploy	0s
	1h 36m 51s

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