clientv3: fix flaky balancer tests

```
TestBalancerUnderServerStopInflightLinearizableGetOnRestart (27.62s)
server_shutdown_test.go:343: context deadline exceeded

TestBalancerUnderServerShutdownDelete (3.22s)
util.go:32: context deadline exceeded
```

Once balancer rewrite is merged, revert https://github.com/coreos/etcd/pull/9555.
