Port unit tests to FreeBSD

The current unit tests are `// +build linux` and are not trivially-enabled on FreeBSD due to dependencies like [`github.com/containerd/continuity/testutil/loopback`](https://github.com/containerd/continuity/blob/master/testutil/loopback/loopback_linux.go).  Before we enable CI (https://github.com/containerd/zfs/issues/44), it'd be useful to get the tests to run.