Refactor trusted resources test code

[Yongxuanzhang]

Test helper functions in https://github.com/tektoncd/pipeline/blob/main/test/trustedresources.go should be refactored into files where they are called, this will improved the readability

1. try to remove the https://github.com/tektoncd/pipeline/blob/main/test/trustedresources.go by adding helper functions to each file

• remove GetUnsignedTask helper function and use var in each file #6799
• remove GetUnsignedPipeline helper function and use var in each file #6800

2. Clean up the test code to meet go style

[Yongxuanzhang]

/assign

[jerop]

/kind cleanup

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

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