Code review comment for ~pwlars/checkbox/+git/support:test-in-lxd

The second commit I just added replaces test-in-lxc.sh with the new test-in-lxd.sh. So anything using test-in-lxc should automatically convert over to it. Landing this, and the corresponding patch to the pmr jobs at https://code.launchpad.net/~pwlars/hwcert-jenkins-jobs/+git/hwcert-jenkins-jobs/+merge/359641 may require some manual help, but we'll see.