Flaky test: TestNewClientWithTimeout

### Description

```
=== FAIL: github.com/docker/docker/pkg/plugins TestNewClientWithTimeout (0.03s)
    client_test.go:167: started remote plugin server listening on: http://127.0.0.1:49311/
    client_test.go:180: assertion failed: error is nil, not os.IsTimeout
```


### docker version

```bash
master @ 23e1af45c67b6931d4942c73652bb9883fe499d8
```
