Find elements through the actor rather than the driver

"Actor::find" is a more robust way to look for elements, as it handles
some exceptions that may be thrown. Therefore, even if the elements are
not actually used and it is only checked whether they exist or not using
the actor is the preferred way when possible (and it also makes it
consistent with the rest of the acceptance tests).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>