test: retry ServiceUnavailable posix_download_test (#8217)

* test: retry ServiceUnavailable posix_download_test

* Fix build

* Fix build

Author: sydney-munro

storagetransfer/posix_download_test.py

@@ -14,12 +14,13 @@
 
 import backoff
 from google.api_core.exceptions import RetryError
+from google.api_core.exceptions import ServiceUnavailable
 from google.cloud.storage import Bucket
 
 import posix_download
 
 
-@backoff.on_exception(backoff.expo, (RetryError,), max_time=60)
+@backoff.on_exception(backoff.expo, (RetryError, ServiceUnavailable,), max_time=60)
 def test_posix_download(
         capsys, project_id: str, job_description_unique: str,
         agent_pool_name: str, posix_root_directory: str,