test(sdk): add placeholder_utils_test.py

[connor-mccarthy]

Description of your changes:
Adds test file which was omitted from #10298.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[connor-mccarthy]

/assign @chensun

[google-oss-prow]

@connor-mccarthy: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kfp-kubernetes-execution-tests	fa5aaf2	link	false	/test kfp-kubernetes-execution-tests
kubeflow-pipelines-sdk-execution-tests	fa5aaf2	link	true	/test kubeflow-pipelines-sdk-execution-tests

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[chensun]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun

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