Fix of TestWorkQueueHealthCheck test

[aLekSer]

Add check that workers actually started before we run main test logic.
If there are no workers live check returns true because 0 workers is
running. And then no error is generated cause workers has not started
(Healthy() return true even if no workers).

How to reproduce the fail seen on cloudbuild:
Add 1 second before Run():

    fun := func() {
        time.Sleep(1 * time.Second)
        wq.Run(1, stop)
    }
    go fun()

[agones-bot]

Build Succeeded 👏

Build Id: 2b39be84-93dd-45f3-8b79-d44393bb230e

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:0.8.0-842b4bc
• image: gcr.io/agones-images/agones-sdk:0.8.0-842b4bc
• image: gcr.io/agones-images/agones-ping:0.8.0-842b4bc
• Linux C++ SDK (build: dev): agonessdk-0.8.0-842b4bc-dev-linux-arch_64.tar.gz
• Linux C++ SDK (build: runtime): agonessdk-0.8.0-842b4bc-runtime-linux-arch_64.tar.gz
• Linux C++ SDK (source): agonessdk-0.8.0-842b4bc-src.zip
• SDK Server: agonessdk-server-0.8.0-842b4bc.zip

A preview of the website (the last 30 builds are retained):

• https://842b4bc-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/568/head:pr_568 && git checkout pr_568
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.8.0-842b4bc