test: mitigate flaky test-https-agent

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

PR-URL: #5939
Fixes: #5938
Refs: #5184
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

test/parallel/test-https-agent.js

@@ -23,8 +23,8 @@ var server = https.Server(options, 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++) {