Add tests for failing remote resolution of Pipelines and Tasks

[abayer]

Changes

Prompted by discussion on #4710, though I should have thought of this earlier!

This can't verify that the ResolutionRequest is handled correctly by its own reconciler, since
that's still in a separate project (for now!), but at least we can be sure that the PipelineRun
and TaskRun reconcilers will respond correctly to a failed ResolutionRequest.

/kind misc

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[abayer]

/assign @vdemeester
/assign @lbernick
/assign @chitrangpatel

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[jerop]

thanks @abayer!

/lgtm

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