test: fix flaky test-memory-usage

abe6a2e introduced a test that verifies that ArrayBuffer
allocations are tracked. However, V8 supports concurrent freeing
of such allocations on background threads, meaning that the results
may be subject to race conditions sometimes.

Disabling concurrent freeing makes the test pass consistently.

Refs: nodejs#31550

test/parallel/test-memory-usage.js

@@ -19,6 +19,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+// Flags: --no-concurrent-array-buffer-freeing
 'use strict';
 const common = require('../common');
 const assert = require('assert');
@@ -41,5 +42,5 @@ if (r.arrayBuffers > 0) {
   assert(after.external - r.external >= size,
          `${after.external} - ${r.external} >= ${size}`);
   assert.strictEqual(after.arrayBuffers - r.arrayBuffers, size,
-                     `${after.arrayBuffers} - ${r.arrayBuffers} >= ${size}`);
+                     `${after.arrayBuffers} - ${r.arrayBuffers} === ${size}`);
 }