client.Kill() should close connections in Test mode

Hi,
For now, if plugin runs in test mode, client will not close its connections, because of this check, that `process == nil` runs early:
https://github.com/hashicorp/go-plugin/blob/master/client.go#L414

This can cause memory leaks in plugin.

Original issue in terraform-provider-aws:
https://github.com/hashicorp/terraform-provider-aws/issues/26130