TestDisk_partitions fails when there are no physical drives mounted

Describe the bug TestDisk_partitions fails when there are no physical drives mounted:

=== RUN   TestDisk_partitions
    disk_test.go:38: error <nil>
    disk_test.go:40: []
    disk_test.go:43: ret is empty
--- FAIL: TestDisk_partitions (0.00s)

To Reproduce Per the Debian bug report, this occurs with the sbuild unshare backend.

Expected behavior The test detects there are no disks and is skipped.

Environment (please complete the following information): Debian sid

Additional context Originally reported as Debian bug #1071190, forwarding upstream.