test(balloon): ensure we gave stats enough time to update before reading

test_balloon_snapshot started being flaky after we changed the logic on
how we wait for the RSS to become stable.
One theory is that we are not waiting enough time for the stats to
refresh, so this change adds a sleep to ensure we have waited enough for
the stats to be "fresh".

Failure:
```
assert 189022208 > 189022208
```

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

tests/integration_tests/functional/test_balloon.py

@@ -522,6 +522,8 @@ def test_balloon_snapshot(microvm_factory, guest_kernel, rootfs):
 
     # Get the stats after we take a snapshot and dirty some memory,
     # then reclaim it.
+    # Ensure we gave enough time for the stats to update.
+    time.sleep(STATS_POLLING_INTERVAL_S)
     latest_stats = microvm.api.balloon_stats.get().json()
 
     # Ensure the stats are still working after restore and show