worker: fix container-sweeper test
worker: fix container-sweeper test

Test started failing more often with upgrade to go 1.14. Error message
is now different.

Signed-off-by: Aidan Oldershaw <30ef7a6e0d016bc846f26d89dacbd23aceded206@pivotal.io>
Co-authored-by: Muntasir Chowdhury <3af480f42ae632f8ec7bb4ce9ac5df283cd495d6@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse