Set tfjob defaults in test utils

[ohmystack]

When creating a tfjob object for testing, the default values should also be filled in using the same defaulter function in the scheme.

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This change is 

[k8s-ci-robot]

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[gaocegege]

/lgtm
/ok-to-test
/approve

Thanks for your contribution! 🎉 👍

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[coveralls]

Coverage remained the same at 76.744% when pulling 09eb600 on ohmystack:defaults-in-test into d2499dc on kubeflow:master.