test: cover DeviceAllow parsing

Add unit coverage for config_parse_device_allow().

Verify valid device paths and subsystem patterns, invalid
specifiers and rights, default permissions, and reset handling.

Follow-up for: 20d52ab60e

TODO.md

@@ -2684,9 +2684,6 @@ SPDX-License-Identifier: LGPL-2.1-or-later
   automatically support reverting back to older OS version images if newer ones
   fail to boot.
 
-- **test/:**
-  - add unit tests for config_parse_device_allow()
-
 - The bind(AF_UNSPEC) construct (for resetting sockets to their initial state)
   should be blocked in many cases because it punches holes in many sandboxes.