test: fix calculations in test-worker-resource-limits

The heap size limit should be the sum of old generation and young
generation size limits, and does not solely depend on the limit
of the old generation.

PR-URL: #42702
Refs: #35711
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: joyeecheung

test/parallel/test-worker-resource-limits.js

@@ -35,10 +35,10 @@ if (!process.env.HAS_STARTED_WORKER) {
 assert.deepStrictEqual(resourceLimits, testResourceLimits);
 const array = [];
 while (true) {
-  // Leave 10% wiggle room here, and 20% on debug builds.
-  const wiggleRoom = common.buildType === 'Release' ? 1.1 : 1.2;
   const usedMB = v8.getHeapStatistics().used_heap_size / 1024 / 1024;
-  assert(usedMB < resourceLimits.maxOldGenerationSizeMb * wiggleRoom);
+  const maxReservedSize = resourceLimits.maxOldGenerationSizeMb +
+                          resourceLimits.maxYoungGenerationSizeMb;
+  assert(usedMB < maxReservedSize);
 
   let seenSpaces = 0;
   for (const { space_name, space_size } of v8.getHeapSpaceStatistics()) {