Add more unit tests for using non-exist key of an object in task step

[chuangw6]

Changes

Prior to this commit, the validation exists to check if individual keys
of an object param being used in task steps indeed exist, but the test
was missed.

In this PR, we add the unit test for that.

/kind misc

Submitter Checklist

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• Has Docs included if any changes are user facing
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Release Notes

Add more unit tests to cover the case where non-exist individual keys of an object are used in task step.

[tekton-robot]

Skipping CI for Draft Pull Request.
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[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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[abayer]

/lgtm

[chuangw6]

/retest