Run CI on both Yakkety and Xenial

Here are the target platforms we intend to support:

 - yakkety, because trunk / distro CI
 - xenial, for autobuilding of xenial ubuntu-core images in Launchpad
 - xenial as a snap, which includes bundled yakkety e2fsprogs and mtools

We're limited though in what we can effectively test in the GitHub CI because of Docker image limitations, which don't allow us to loop mount file systems. The e2fsprogs version in Xenial doesn't support prepopulating the ext4 file system (i.e. the -d option), so we have to mount+cp there. This requires us to skip some tests.

We can and should run the full test suite under Yakkety though, and this is the top priority. Currently, CI only runs with a Xenial image, and there only with a partial test suite run.