tests: unflake ud system test to only run in prod and hmac sample test (

#1353)

* test: test universe domain client only in prod

* unflake hmac snippet test

samples/snippets/hmac_samples_test.py

@@ -64,7 +64,10 @@ def new_hmac_key():
     if not hmac_key.state == "INACTIVE":
         hmac_key.state = "INACTIVE"
         hmac_key.update()
-    hmac_key.delete()
+    try:
+        hmac_key.delete()
+    except google.api_core.exceptions.BadRequest:
+        pass
 
 
 def test_list_keys(capsys, new_hmac_key):

tests/system/test_client.py

@@ -186,6 +186,10 @@ def test_download_blob_to_file_w_etag(
     assert buffer.getvalue() == payload
 
 
+@pytest.mark.skipif(
+    _helpers.is_api_endpoint_override,
+    reason="Credentials not yet supported in preprod testing.",
+)
 def test_client_universe_domain(
     universe_domain_client,
     test_universe_location,