dm_test.cpp: DeleteDeviceWithTimeout asserts that unique path is deleted

Before this patch, DeleteDeviceWithTimeout was checking that the dev
node (i.e. /dev/block/dm-XX) is deleted after the call to DeleteDevice
API. Since ueventd first deletes the symlinks that correspond to a
device and only then deletes this device node, this assertion introduced
a race condition (DeleteDevice API waits for the symlink to be deleted).

This patch changes the DeleteDeviceWithTimeout test to check that unique
path of the device has been deleted.

Bug: 318425605
Test: presubmit
Change-Id: I3fd9de507c75bcf6ac1350fa0b8adfdb5a2e89e8