test(images): let the settle loops yield so a finished walk leaves the set (#348)

On CI (ubuntu and macOS py3.12) the runner's watchdog dumped
test_a_root_that_recovers_warns_again_on_its_next_failure spinning in
_settle: asyncio.gather over tasks that are already done returns without
yielding, so the set's discard callback, queued when the walk finished,
never ran and the loop never ended (wait_for could not fire either).
Delaying discard by one loop turn reproduces it locally; both settle
loops now yield one turn after each gather.

tests/test_api_images.py

@@ -2431,6 +2431,9 @@ class TestSharedListing(ImageAPITestCase):
         async def settle():
             while self.api._listing_tasks:
                 await asyncio.gather(*list(self.api._listing_tasks))
+                # A finished walk leaves the set in a done callback, which needs
+                # a loop turn; gather over finished tasks does not yield one.
+                await asyncio.sleep(0)
 
         await asyncio.wait_for(settle(), _HANG_LIMIT)
 

tests/test_api_workspace_images.py

@@ -196,6 +196,9 @@ class TestWorkspaceImages(ImageAPITestCase):
             await self.get()  # served at once; one background walk runs
             while self.api._listing_tasks:
                 await asyncio.gather(*list(self.api._listing_tasks))
+                # A finished walk leaves the set in a done callback, which needs
+                # a loop turn; gather over finished tasks does not yield one.
+                await asyncio.sleep(0)
             _, again = await self.get()
         self.assertEqual(scan.call_count, 1)
         self.assertIs(record.files, files)