test: minimize test-http-get-pipeline-problem

[Trott]

Pull Request check-list

Please make sure to review and check all of these items:

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?
• If this change fixes a bug (or a performance problem), is a regression
  test (or a benchmark) included?
• Is a documentation update included (if this change modifies
  existing APIs, or introduces new ones)?

Affected core subsystem(s)

test, http

Description of change

Reduce resoures required by test. Fixes test flakiness on Raspberry Pi in CI.

Clarify comment explaining source of test and what the test is looking
for.

Fixes: #5725

[Trott]

Stress test showing flakiness on current master: https://ci.nodejs.org/job/node-stress-single-test/560/nodes=pi2-raspbian-wheezy/console

[Trott]

For context:

Here's a gist from @mikeal who originally reported the bug: https://gist.githubusercontent.com/mikeal/864727/raw/561f3b665df0faff6d3ee9601ef31cc3caf0376f/gistfile1.js

That code fails on Node.js 0.6.0 and succeeds in 0.6.21.

If we change it to this, it still failsin Node.js 0.6.0 and succeeds in 0.6.21:

var http = require('http');
var fs = require('fs');
var parseUrl = require('url').parse;
http.globalAgent.maxSockets = 1;
var i = 10;
while(i > 0) {
(function () {
var x = i;
var u = parseUrl("http://farm4.static.flickr.com/3557/3670453345_c38d482efb_m.jpg")
var c = http.request(
{ host:u.hostname
, port:u.port
, method: 'GET'
, path: u.pathname
, headers: {host:u.hostname}
}
, function (res) {
console.log(x)
res.pipe(fs.createWriteStream(x + ".jpg"))
})
c.end();
i--;
})()
}

The key is that http.globalAgent.maxSockets needs to be less than i to trip the bug. So, that's why we are able to reduce the number of sockets in this test the way we do in this PR.

[Trott]

Stress test to hopefully show that this is not flaky. Hope I didn't typo in the parameters...

https://ci.nodejs.org/job/node-stress-single-test/563/nodes=pi2-raspbian-wheezy/console

[Trott]

Bump. /cc @nodejs/testing

[santigimeno]

LGTM

[cjihrig]

LGTM

[jasnell]

LGTM

[jasnell]

Landed in ccb7b45