Investigate flaky test-https-agent

[Trott]

Example failure on pi2-raspbian-wheezy: https://ci.nodejs.org/job/node-test-binary-arm/1511/RUN_SUBSET=1,nodes=pi2-raspbian-wheezy/console

[Trott]

Looks like this is basically the same as what was fixed in a similar test by bbf4621