volume/overlay: add driver import to tests
volume/overlay: add driver import to tests

previously, we had `driver` coming as a dot import. Once that was
removed, the tests didn't compile anymore.

Here I bring driver back, but as a regular import this time.

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/baggageclaim,concourse/baggageclaim