chore: add failing test of slotToVal memory leak for watchPromise

Liveslots has a bug (#10757) which leaks slotToVal entries when a
tracked Promise is still being held in virtual data (e.g. a
merely-virtual MapStore) at the time it becomes settled. This is
triggered by `watchPromise` because of the order in which we attach
two handlers: one which notices the resolution and is inhibited from
deleting the slotToVal entry, and a second which removes the Promise
from the (virtual) `promiseRegistrations` collection (thus enabling
the deletion). For any watched Promise that is resolved, we leave a
`slotToVal` entry (with an empty WeakRef) in RAM until the end of the
incarnation.

This commit adds a test.failing to demonstrate the presence of the
bug. Each time we watch and then resolve a promise, the slotToVal
table grows by one entry.

refs #10756

packages/swingset-liveslots/test/watch-promise.test.js

@@ -0,0 +1,42 @@
+import test from 'ava';
+
+import { Far } from '@endo/marshal';
+import { makePromiseKit } from '@endo/promise-kit';
+import { setupTestLiveslots } from './liveslots-helpers.js';
+
+const build = vatPowers => {
+  const { VatData } = vatPowers;
+  const { makeKindHandle, defineDurableKind, watchPromise } = VatData;
+
+  const kh = makeKindHandle('handler');
+  const init = () => ({});
+  const behavior = {
+    onFulfilled: _value => 0,
+    onRejected: _reason => 0,
+  };
+  const makeHandler = defineDurableKind(kh, init, behavior);
+
+  return Far('root', {
+    async run() {
+      const pr = makePromiseKit();
+      const handler = makeHandler();
+      watchPromise(pr.promise, handler);
+      pr.resolve('ignored');
+    },
+  });
+};
+
+test.failing('watched local promises should not leak slotToVal entries', async t => {
+  const { dispatchMessage, testHooks } = await setupTestLiveslots(
+    t,
+    build,
+    'vatA',
+  );
+  const { slotToVal } = testHooks;
+  const initial = slotToVal.size;
+
+  await dispatchMessage('run');
+  t.is(slotToVal.size, initial);
+  await dispatchMessage('run');
+  t.is(slotToVal.size, initial);
+});