storage: fix failing system tests (#7714)

* Add additional object deletion

* Add versions to list_blobs during deletion

* Fix retry call to empty bucket

storage/tests/system.py

@@ -51,15 +51,11 @@ def _bad_copy(bad_request):
 
 
 def _empty_bucket(bucket):
-    """Empty a bucket of all existing blobs.
-
-    This accounts (partially) for the eventual consistency of the
-    list blobs API call.
-    """
-    for blob in bucket.list_blobs():
+    """Empty a bucket of all existing blobs (including multiple versions)."""
+    for blob in bucket.list_blobs(versions=True):
         try:
             blob.delete()
-        except exceptions.NotFound:  # eventual consistency
+        except exceptions.NotFound:
             pass
 
 
@@ -87,6 +83,7 @@ def setUpModule():
 def tearDownModule():
     errors = (exceptions.Conflict, exceptions.TooManyRequests)
     retry = RetryErrors(errors, max_tries=9)
+    retry(_empty_bucket)(Config.TEST_BUCKET)
     retry(Config.TEST_BUCKET.delete)(force=True)