TestDisk_partitions fails when there are no physical drives mounted

[gibmat]

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.

[shirou]

Thank you for the report. I believe this test is necessary to detect the issue where drives are no longer recognized. The environment where physical drives do not exist is quite rare, so I don't think we can remove this test to accommodate that scenario.