ensure setup_test_user has been called before using test_user

when tempest run in one process, setup_test_user can be called in
other tests. But when tempest run in multiple process, we should
ensure it has been called before using test_user in every test.

Part of blueprint speed-up-tempest

Change-Id: If5aadf0852c578fb198f49be3604d6cbe24155f9
Merge "ensure setup_test_user has been called before using test_user"