test: fix napi test_reference for recent V8

PR-URL: https://github.com/nodejs/node/pull/12864
Ref: https://github.com/nodejs/node/pull/12551#issuecomment-297949361
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/addons-napi/test_reference/test.js

@@ -33,19 +33,6 @@ assert.strictEqual(test_reference.finalizeCount, 0);
   assert.strictEqual(test_reference.finalizeCount, 1);
 }
 
-{
-  // Weak reference
-  let value = test_reference.createExternalWithFinalize();
-  assert.strictEqual(test_reference.finalizeCount, 0);
-  test_reference.createReference(value, 0);
-  assert.strictEqual(test_reference.referenceValue, value);
-  value = null;
-  global.gc(); // Value should be GC'd because there is only a weak ref
-  assert.strictEqual(test_reference.referenceValue, undefined);
-  assert.strictEqual(test_reference.finalizeCount, 1);
-  test_reference.deleteReference();
-}
-
 {
   // Strong reference
   let value = test_reference.createExternalWithFinalize();
@@ -85,3 +72,19 @@ assert.strictEqual(test_reference.finalizeCount, 0);
   global.gc(); // Value was already GC'd
   assert.strictEqual(test_reference.finalizeCount, 1);
 }
+
+{
+  // Weak reference
+  let value = test_reference.createExternalWithFinalize();
+  assert.strictEqual(test_reference.finalizeCount, 0);
+  test_reference.createReference(value, 0);
+  assert.strictEqual(test_reference.referenceValue, value);
+  value = null;
+  setImmediate(common.mustCall(() => {
+    // This test only works if gc() is called from an immediate callback.
+    global.gc(); // Value should be GC'd because there is only a weak ref
+    assert.strictEqual(test_reference.referenceValue, undefined);
+    assert.strictEqual(test_reference.finalizeCount, 1);
+    test_reference.deleteReference();
+  }));
+}