Use apt pinning instead of APT::Default-Release

Apt verifies that the value of APT::Default-Release exists in sources.list, and
this breaks assumptions in certain tests which expect to be able to construct
arbitrary sources.lists and work with them.

Pin files aren't subject to this validation, but are otherwise equivalent -
let's use one of those.

Closes: #893754

Based on an initial patch by Iain Lane <iain.lane@canonical.com>