chore(swingset): improve test-transcript

Update test-transcript.js to wait for the combined run-queue and GC-actions
queue to drain, now that c.run()/c.step() executes GC actions first. Also
shutdown the controller properly after each call to buildTrace().

This test exhibits odd GC behavior when:

* it lacks the wait for `gcActions.length`
* and uses the default 'local' vat worker
* liveslots calls `gcAndFinalize()` only once

This suggests that our `gcAndFinalize()` is insufficient, at least on
Node.js. See #3240 for more details.

packages/SwingSet/test/test-transcript.js

@@ -10,12 +10,13 @@ import { buildVatController, loadBasedir } from '../src/index';
 
 async function buildTrace(c, storage) {
   const states = [];
-  while (c.dump().runQueue.length) {
+  while (c.dump().runQueue.length && c.dump().gcActions.length) {
     states.push(getAllState(storage));
     // eslint-disable-next-line no-await-in-loop
     await c.step();
   }
   states.push(getAllState(storage));
+  await c.shutdown();
   return states;
 }