test: speed up test-net-reconnect-error

The test takes 50 seconds on some of the project's Windows CI
infrastructure. Reducing the test repetitions from 50 to 20 trims that
to about 20 seconds. Tests will timeout at 60 seconds, so this helps
keep the test reliable. (There was a timeout on CI today when testing an
unrelated code change.)

Refs: nodejs#7827 (comment)

test/parallel/test-net-reconnect-error.js

@@ -2,7 +2,7 @@
 var common = require('../common');
 var net = require('net');
 var assert = require('assert');
-var N = 50;
+var N = 20;
 var client_error_count = 0;
 var disconnect_count = 0;