Allow tests to run one at at a time, and halt on error.
Allow tests to run one at at a time, and halt on error.

Revision created by MOE tool push_codebase.
MOE_MIGRATION=6511

aozarov/appengine-gcs-client,aozarov/appengine-gcs-client,GoogleCloudPlatform/appengine-gcs-client,GoogleCloudPlatform/appengine-gcs-client,aozarov/appengine-gcs-client,GoogleCloudPlatform/appengine-gcs-client