Refactor pipelinerun metrics tests

[afrittoli]

Changes

Refactor the tests for the new pipelinerun metric as a matrix test. Ensure we test the case of non-running pipelineruns.

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

Test name	Commit	Details	Required	Rerun command
pull-tekton-pipeline-build-tests	a8724fb	link	true	/test pull-tekton-pipeline-build-tests

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