test: mitigate flaky test-http-agent

Reduce number of clients from 100 to 16 as Raspberry Pi in CI starts to
exhibit flakiness around 22 or so clients.

Fixes: https://github.com/nodejs/node/issues/5184
PR-URL: https://github.com/nodejs/node/pull/5346
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/parallel.status

@@ -10,7 +10,6 @@ prefix parallel
 test-tick-processor     : PASS,FLAKY
 
 [$system==linux]
-test-http-agent         : PASS,FLAKY
 test-tick-processor     : PASS,FLAKY
 
 [$system==macos]

test/parallel/test-http-agent.js

@@ -9,8 +9,8 @@ var server = http.Server(function(req, res) {
 });
 
 var responses = 0;
-var N = 10;
-var M = 10;
+var N = 4;
+var M = 4;
 
 server.listen(common.PORT, function() {
   for (var i = 0; i < N; i++) {