Investigate flaky test-http-regr-gh-2928

[Trott]

Example failure:

• smartos14-32

Stress test confirmation: https://ci.nodejs.org/job/node-stress-single-test/528/nodes=smartos14-32/console

Ref: #4830

[Trott]

/cc @mscdex

[Trott]

ARGH! This is the SmartOS bug that we actually patched a workaround for in test.py but the workaround is insufficient for tests that have a lot of connections. See 41519fd

[Trott]

Ref: #3960

[Trott]

Ref: #3941

[Trott]

I think I have a working fix. Will run a Raspberry Pi stress test just to make sure I'm not re-introducing the issue on that platform by solving it for SmartOS. If all goes well, PR tomorrow.