kv/etcd: Fix race condition within unit test for Watch method #10884

Job	Run time
lint	4m 38s
lint	4m 38s
CodeQL	6m 52s
CodeQL	6m 52s
build	4m 16s
build	4m 16s
test	15m 51s
test	16m 7s
test-no-race	7m 34s
test-no-race	7m 34s
integration-configs-db	2m 59s
integration-configs-db	2m 59s
integration (requires_docker)	7m 19s
integration (requires_docker)	7m 19s
integration (integration_alertmanager)	4m 24s
integration (integration_alertmanager)	4m 24s
integration (integration_backward_compatibility)	11m 1s
integration (integration_backward_compatibility)	11m 1s
integration (integration_memberlist)	7m 41s
integration (integration_memberlist)	7m 41s
integration (integration_querier)	15m 7s
integration (integration_querier)	15m 7s
integration (integration_ruler)	5m 52s
integration (integration_ruler)	5m 52s
integration (integration_query_fuzz)	5m 4s
integration (integration_query_fuzz)	5m 4s
deploy_website	0s
deploy_website	0s
deploy	0s
deploy	0s
	3h 17m 32s

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