Increase timeout in Conversion Integration Test to Prevent Flake

[JeromeJu]

Changes

This commit increases the timeout length in conversion_test to prevent flake in that pipeline could fail waiting for the TaskRun to get done given the previous timeout.

Potential flake observed at:
https://prow.tekton.dev/view/gs/tekton-prow/pr-logs/pull/tektoncd_pipeline/6022/pull-tekton-pipeline-integration-tests/1617909197544886272

    stream.go:254: D 15:51:47.598 tekton-pipelines-controller-5cc86f7df8-tcwgv [github.com.tektoncd.pipeline.pkg.reconciler.pipelinerun.Reconciler] [arendelle-xrpr8/git-resolver-clone-vktppyly] tracerProvider doesn't provide a traceId, tracing is disabled
=== CONT  TestTaskRunCRDConversion
    conversion_test.go:869: Failed waiting for v1beta1 TaskRun done: "task-run-c-r-d-conversion-wskyiehs" failed
    panic.go:500: ############################################
    panic.go:500: ### Dumping objects from arendelle-msvkl ###
    panic.go:500: ############################################

/kind misc

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [n/a] Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
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  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
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Release Notes

NONE

[Yongxuanzhang]

/lgtm
the error msg is TaskRun "task-run-c-r-d-conversion-wskyiehs" failed to finish within "20s", it makes sense to increase the timeout to avoid this failure

[jerop]

Thanks @JeromeJu

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