Add build tags so that the test only runs on Darwin

[tstromberg]

Fixes Linux / Travis build issue caused as a side-effect of me force-merging #5833:

# k8s.io/minikube/pkg/minikube/registry/drvs/hyperkit [k8s.io/minikube/pkg/minikube/registry/drvs/hyperkit.test]
pkg/minikube/registry/drvs/hyperkit/hyperkit_test.go:55:15: undefined: splitHyperKitVersion
pkg/minikube/registry/drvs/hyperkit/hyperkit_test.go:85:15: undefined: convertVersionToDate
pkg/minikube/registry/drvs/hyperkit/hyperkit_test.go:119:18: undefined: isNewerVersion

[k8s-ci-robot]

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@dad37ae). Click here to learn what that means.
The diff coverage is n/a.

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  Hits              ?     3071           
  Misses            ?     4887           
  Partials          ?      403

[govargo]

I understand why my unit test failed.
Thank you.