test: use checkIfCollectable() in diganostics channel leak test

Previously it only checks that the heap memory usage is down after
global.gc() returns, which is unreliable because global.gc() does
not guarantee that the heap memory usage would go down even when
there is no leak (it could go up due to promotion, or it only
kicks off marking). Use the more reliable checkIfCollectable()
utility instead.

Author: joyeecheung

test/parallel/test-diagnostics-channel-memory-leak.js

@@ -1,24 +1,18 @@
-// Flags: --expose-gc
+// Flags: --max-old-space-size=16
 'use strict';
 
 // This test ensures that diagnostic channel references aren't leaked.
 
 require('../common');
-const { ok } = require('assert');
-
-const { subscribe, unsubscribe } = require('diagnostics_channel');
+const { checkIfCollectable } = require('../common/gc');
+const { subscribe, unsubscribe, channel } = require('diagnostics_channel');
 
 function noop() {}
 
-const heapUsedBefore = process.memoryUsage().heapUsed;
-
-for (let i = 0; i < 1000; i++) {
-  subscribe(String(i), noop);
-  unsubscribe(String(i), noop);
-}
-
-global.gc();
-
-const heapUsedAfter = process.memoryUsage().heapUsed;
-
-ok(heapUsedBefore >= heapUsedAfter);
+let i = 0;
+checkIfCollectable(() => {
+  const key = new Array(1024 * 1024).fill(i++).join('\n');
+  subscribe(key, noop);
+  unsubscribe(key, noop);
+  return channel(key);
+}, 4096, 1024);