Investigate flaky test-http-hightwatermark

[Trott]

• Version: 10.0.0-pre
• Platform: macOS
• Subsystem: test

I've seen failures locally on my macOS laptop with make test of test-http-highwatermark. I'm able to reproduce it pretty easily with the right set of options:

$ tools/test.py -j 96 --repeat 192 test/parallel/test-http-hightwatermark.js 
=== release test-http-hightwatermark ===                    
Path: parallel/test-http-hightwatermark
Mismatched <anonymous> function calls. Expected exactly 1, actual 0.
    at Object.exports.mustCall (/Users/trott/io.js/test/common/index.js:496:10)
    at Server.<anonymous> (/Users/trott/io.js/test/parallel/test-http-hightwatermark.js:21:41)
    at Server.emit (events.js:131:13)
    at parserOnIncoming (_http_server.js:617:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:115:23)
Mismatched <anonymous> function calls. Expected exactly 1, actual 0.
    at Object.exports.mustCall (/Users/trott/io.js/test/common/index.js:496:10)
    at Server.<anonymous> (/Users/trott/io.js/test/parallel/test-http-hightwatermark.js:28:43)
    at Server.emit (events.js:131:13)
    at parserOnIncoming (_http_server.js:617:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:115:23)
Command: out/Release/node /Users/trott/io.js/test/parallel/test-http-hightwatermark.js
=== release test-http-hightwatermark ===                   
Path: parallel/test-http-hightwatermark
Mismatched <anonymous> function calls. Expected exactly 1, actual 0.
    at Object.exports.mustCall (/Users/trott/io.js/test/common/index.js:496:10)
    at Server.<anonymous> (/Users/trott/io.js/test/parallel/test-http-hightwatermark.js:21:41)
    at Server.emit (events.js:131:13)
    at parserOnIncoming (_http_server.js:617:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:115:23)
Mismatched <anonymous> function calls. Expected exactly 1, actual 0.
    at Object.exports.mustCall (/Users/trott/io.js/test/common/index.js:496:10)
    at Server.<anonymous> (/Users/trott/io.js/test/parallel/test-http-hightwatermark.js:28:43)
    at Server.emit (events.js:131:13)
    at parserOnIncoming (_http_server.js:617:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:115:23)
Command: out/Release/node /Users/trott/io.js/test/parallel/test-http-hightwatermark.js
[00:08|% 100|+ 190|-   2]: Done
$