fix: make test-activityhash.. use xs-worker

I'm seeing CI failures under Node-18 in this test, presumeably because
we get GC variation between the first run and the replay. Super
annoying, and not germane to what this test is supposed to be
executing. The `test.serial` wrapper wasn't enough to fix it, so I've
just switched the test to only use xs-worker.

refs #3240

packages/SwingSet/test/test-activityhash-vs-start.js

@@ -11,14 +11,16 @@ const TimerSrc = new URL(
   import.meta.url,
 ).pathname;
 
-// all tests that are sensitive to GC timing (which means anything that
-// exercises transcript replay or looks at activityHash) need to use
-// test.serial or config.defaultManagerType='xsnap', until we figure out why
-// gcAndFinalize sometimes doesn't work (details in #3240 and #4617)
+// all tests that are sensitive to GC timing (which means anything
+// that exercises transcript replay or looks at activityHash) need to
+// use test.serial or config.defaultManagerType='xs-worker', until we
+// figure out why gcAndFinalize sometimes doesn't work (details in
+// #3240 and #4617). And even test.serial doesn't seem to be enough.
 
-test.serial('restarting kernel does not change activityhash', async t => {
+test('restarting kernel does not change activityhash', async t => {
   const sourceSpec = new URL('vat-empty-setup.js', import.meta.url).pathname;
   const config = {
+    defaultManagerType: 'xs-worker',
     bootstrap: 'bootstrap',
     vats: {
       bootstrap: {
@@ -91,14 +93,14 @@ test.serial('restarting kernel does not change activityhash', async t => {
   t.is(c1ah, c2ah);
 });
 
-test.serial('comms initialize is deterministic', async t => {
+test('comms initialize is deterministic', async t => {
   // bug #3726: comms was calling vatstoreGet('initialize') and
   // vatstoreSet('meta.o+0') during the first message after process restart,
   // which makes it a nondeterministic function of the input events.
 
   const sourceSpec = new URL('vat-activityhash-comms.js', import.meta.url)
     .pathname;
-  const config = {};
+  const config = { defaultManagerType: 'xs-worker' };
   config.bootstrap = 'bootstrap';
   config.vats = { bootstrap: { sourceSpec } };
   const { kernelStorage: ks1, debug: debug1 } = initSwingStore();