Update test-infra dependency

[adrcunha]

Includes:

• fixes/updates so e2e-test.sh run on OS X;
• fixes to test environment setup;
• fixes to library.sh in order to have some hack scripts working again;
• better checking of PRs where presubmit tests are unnecessary;

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adrcunha, mattmoor

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