Flaky Test: `TestStoreGateway_InitialSyncWithWaitRingStability`

[56quarters]

Describe the bug

The TestStoreGateway_InitialSyncWithWaitRingStability is flaky. See this CI run:

 --- FAIL: TestStoreGateway_InitialSyncWithWaitRingStability (140.66s)
    --- FAIL: TestStoreGateway_InitialSyncWithWaitRingStability/shuffle_sharding_strategy,_20_gateways,_RF_=_3,_SS_=_3_(bucket_index_enabled_=_false) (32.45s)
        gateway_test.go:300: random generator seed: 1623962900927129822
        gateway_test.go:347: 
            	Error Trace:	gateway_test.go:347
            	Error:      	Received unexpected error:
            	            	invalid service state: Failed, expected: Running, failure: unable to start store-gateway dependencies: not healthy, 0 terminated, 1 failed: [register instance in the ring: failed to CAS store-gateway]
            	Test:       	TestStoreGateway_InitialSyncWithWaitRingStability/shuffle_sharding_strategy,_20_gateways,_RF_=_3,_SS_=_3_(bucket_index_enabled_=_false)
FAIL
FAIL	github.com/cortexproject/cortex/pkg/storegateway	219.005s

[pracucci]

failed to CAS store-gateway

Maybe caused by an high concurrency on the in-memory consul backend we use in unit tests?

[pstibrany]

seen again: https://github.com/cortexproject/cortex/runs/2876052593

[pracucci]

I'm going to investigate this.