[SDK] test: add unit test for list_jobs method of the training_client

[seanlaii]

What this PR does / why we need it:
New unit test for list_jobs function

To verify run:

pytest -v sdk/python/kubeflow/training/api/training_client_test.py -k test_list_jobs

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2161

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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