standalone mode. Since the test is meant to exercise how the metadata
tool interacts with xAOD::TEvent. And it's only in standalone mode that
xAOD::TEvent drives the metadata tools. (In Athena it's Athena's
event loop manager that takes care of firing the incidents that the
metadata tools respond to. And that's not something that a unit test
is supposed to exercise.)

Former-commit-id: 7bcd87cd64ecceb35d3d6718e262f88840133655