Update MPIJob unit tests to use spec.runPolicy.cleanPodPolicy

[cheimu]

What this PR does / why we need it:
This pr update mpijob unit tests: in the past, unit tests uses spec.cleanPodPolicy to test gc logic, but now in training-operator framework, spec.runPolicy.CleanPodPolicy is the standard, so it is more suit to use this field in unit tests as well.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #1555

Checklist:

• Docs included if any changes are user facing

[aws-kf-ci-bot]

Hi @cheimu. Thanks for your PR.

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[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheimu, terrytangyuan

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[cheimu]

@terrytangyuan Hi Terry, It's weird, I don't know why tfjobs tests got failed for this pr. As you can see, except MPIJobs, this pr has nothing else been modified, so I don't know why this could lead to other jobs' unit tests failed. Any ideas? :/ Thank you in advance!

[google-oss-prow]

New changes are detected. LGTM label has been removed.

[coveralls]

Pull Request Test Coverage Report for Build 2012430626

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 36.815%

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Totals
Change from base Build 1996927612:	0.0%
Covered Lines:	2277
Relevant Lines:	6185

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💛 - Coveralls

[cheimu]

@terrytangyuan Hi Terry, It's weird, I don't know why tfjobs tests got failed for this pr. As you can see, except MPIJobs, this pr has nothing else been modified, so I don't know why this could lead to other jobs' unit tests failed. Any ideas? :/ Thank you in advance!

Okay... Never minds, the unit tests got passed in the end :D