test: update persistence agent unit tests. Part of #388

[evan-hataishi]

Which issue is resolved by this Pull Request:
Related #388

Description of your changes:
Update persistence agent unit tests to remove argo related code.

Environment tested:

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• Tekton Version (use tkn version):
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• OS (e.g. from /etc/os-release):

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[evan-hataishi]

/hold

Still need to update persistence worker and workflow_saver.

[evan-hataishi]

/unhold

[Tomcli]

it looks good to me other than the typo. Just listing some TODOs after #395

1. run go test -v -cover ./backend/src/agent/... unit tests in the new travis job

[Tomcli]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evan-hataishi, Tomcli

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