test: harden 'bucket_cmek' systests against EC (#514)

Closes #513.

Author: tseaver

tests/system/_helpers.py

@@ -54,8 +54,13 @@ def _no_event_based_hold(blob):
     return not blob.event_based_hold
 
 
+def _has_kms_key_name(blob):
+    return blob.kms_key_name is not None
+
+
 retry_bad_copy = RetryErrors(exceptions.BadRequest, error_predicate=_bad_copy)
 retry_no_event_based_hold = RetryInstanceState(_no_event_based_hold)
+retry_has_kms_key_name = RetryInstanceState(_has_kms_key_name)
 
 
 def unique_name(prefix):

tests/system/test_kms_integration.py

@@ -229,7 +229,7 @@ def test_blob_upload_w_bucket_cmek_enabled(
     blob.upload_from_string(payload)
     blobs_to_delete.append(blob)
 
-    _helpers.retry_429_harder(blob.reload)()
+    _helpers.retry_429_harder(_helpers.retry_has_kms_key_name(blob.reload))()
     # We don't know the current version of the key.
     assert blob.kms_key_name.startswith(kms_key_name)