Fix test not waiting for cluster member to be ready

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

cluster/cluster_test.go

@@ -87,6 +87,7 @@ func testJoinLeave(t *testing.T) {
 	)
 	require.NoError(t, err)
 	go p2.Settle(context.Background(), 0*time.Second)
+	p2.WaitReady()
 
 	require.Equal(t, 2, p.ClusterSize())
 	p2.Leave(0 * time.Second)