test: fix pummel/test-net-connect-memleak

A loop that generates a long array is resulting in a RangeError. Moving
to Array.prototype.fill() along with the ** operator instead of using a
loop fixes the issue.

PR-URL: https://github.com/nodejs/node/pull/21658
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/pummel/test-net-connect-memleak.js

@@ -37,8 +37,7 @@ let before = 0;
 {
   // 2**26 == 64M entries
   global.gc();
-  let junk = [0];
-  for (let i = 0; i < 26; ++i) junk = junk.concat(junk);
+  const junk = new Array(2 ** 26).fill(0);
   before = process.memoryUsage().rss;
 
   net.createConnection(common.PORT, '127.0.0.1', function() {