fix(swingset): force flaky GC test to use XS only

Sometimes, for reasons we don't entirely understand, Node.js doesn't
garbage-collect objects when we tell it to, and we get flaky
GC-checking tests. This applies our usual fix, which is to only run
those tests under XS.

refs #3240
refs #5575
fixes #9089

packages/SwingSet/test/test-gc-kernel.js

@@ -1180,6 +1180,7 @@ test.serial('device transfer', async t => {
     };
   }
   const config = {
+    defaultManagerType: 'xs-worker', // Avoid local vat nondeterminism
     vats: {
       bootstrap: vatpath('bootstrap-gc.js'),
       left: vatpath('vat-left-gc.js'),