Add missing tests for pipelinerun_defaults and taskrun_defaults

[shuheiktgw]

Changes

Just added missing tests for pipelinerun_defaults and taskrun_defaults! Thanks for your review! 😄

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task and TaskRun to build and release this image

[vdemeester]

/ok-to-test
This is gonna conflict with #1083 😝

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/pipelinerun_defaults.go	0.0%	75.0%	75.0
pkg/apis/pipeline/v1alpha1/taskrun_defaults.go	0.0%	83.3%	83.3

[dlorenc]

/lgtm

[dlorenc]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dlorenc, shuheiktgw

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