Investigate flaky parallel/test-debug-port-cluster

[Trott]

• Version: master
• Platform: ppce-ubuntu1404
• Subsystem: debugger

Example failure on CI: https://ci.nodejs.org/job/node-test-commit-plinux/2460/nodes=ppcbe-ubuntu1404/tapTestReport/test.tap-181/

/cc @bnoordhuis

[Trott]

/cc @nodejs/testing

[mscdex]

I just saw this on FreeBSD too FWIW: https://ci.nodejs.org/job/node-test-commit-freebsd/2478/nodes=freebsd10-64/tapTestReport/test.tap-373/

[Trott]

And SmartOS: https://ci.nodejs.org/job/node-test-commit-smartos/2496/nodes=smartos14-64/console

Something is definitely wrong with that test....

[Trott]

Er...uh...or perhaps equally likely, something is wrong with the code base but the test only catches it intermittently...

[Trott]

This appears to fix the flakiness in this test: #6769