Unexpected backend.exe processes after test-server

Description

For me, on Linux, running `make test-server` leaves some "backend.exe" processes running out of tmp directory. I have 18 of them currently. Coming from here:
[mattermost/mattermost-server/app/post_test.go](https://github.com/mattermost/mattermost-server/blob/1e89ef73143e2ba9ff5217ec44da947400635b04/app/post_test.go#L75)
```go
func main() {
plugin.ClientMain(&MyPlugin{})
}
`, `{"id": "testrejectfirstpost", "backend": {"executable": "backend.exe"}}`, "testrejectfirstpost", th.App)

pendingPostId := model.NewId()
post, err := th.App.CreatePostAsUser(&model.Post{
```
Anyone else seeing this?

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Ian Whitlock

QA Assignee

Lindy Isherwood

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Jesse Hallam

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