test: make test-http-agent-maxsockets robust

On a slow/busy machine, `test-http-agent-maxsockets` can fail if the
test takes longer than 5 seconds because that is the default value for
`server.keepAliveTimeout`. Disable `keepAliveTimeout` to make the test
robust.

PR-URL: #15192
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/parallel/test-http-agent-maxsockets.js

@@ -15,6 +15,8 @@ const server = http.createServer(common.mustCall((req, res) => {
   res.end('hello world');
 }, 2));
 
+server.keepAliveTimeout = 0;
+
 function get(path, callback) {
   return http.get({
     host: 'localhost',