Fix integration TestSingleBinaryWithMemberlistScaling flaking.

This appears to be highlighting an issue - raised as cortexproject#4360.
This change just stops the test flaking until it can be fixed.

Signed-off-by: Steve Simpson <steve.simpson@grafana.com>

integration/integration_memberlist_single_binary_test.go

@@ -197,6 +197,13 @@ func TestSingleBinaryWithMemberlistScaling(t *testing.T) {
 		c := instances[i]
 		instances = instances[:i]
 		stop.Go(func() error { return s.Stop(c) })
+
+		// TODO(#4360): Remove this when issue is resolved.
+		//   Wait until memberlist for all nodes has recognised the instance left.
+		//   This means that we will not gossip tombstones to leaving nodes.
+		for _, c := range instances {
+			require.NoError(t, c.WaitSumMetrics(e2e.Equals(float64(len(instances))), "memberlist_client_cluster_members_count"))
+		}
 	}
 	require.NoError(t, stop.Wait())