autopkgtest broken on deb822 only testbeds

[Skia]

Running apt-clone autopkgtest on a deb822-only such as a fresh Noble image breaks the tests, because they test for the presence of `/etc/apt/sources.list` on the system they run (not the embedded mocked test data).
Checking for another file would fix this, or removing the "real system" test, since this is heavily dependent on the testbed, and thus quite brittle.

Here is an example of broken run: https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/arm64/a/apt-clone/20240419_132003_66310@/log.gz

[Skia (hyask)]

Here is a debdiff fixing the test by checking `/etc/passwd` instead of `/etc/apt/sources.list`.

[Nick Rosbrook (enr0n)]

Hm, it doesn't look like the tested function (_find_unowned_in_etc()) is actually used anywhere besides this test...

But that's something to sort out in the next devel series I guess. This should do the trick for now.

[Launchpad Janitor (janitor)]

This bug was fixed in the package apt-clone - 0.4.3+nmu2ubuntu2

---------------
apt-clone (0.4.3+nmu2ubuntu2) noble; urgency=medium

  * Fix tests on deb822-only system (LP: #2062622)
    On those systems, `/etc/apt/sources.list` doesn't exist. Let's check for
    `/etc/passwd` instead in the real system test.

 -- Florent 'Skia' Jacquet <email address hidden> Fri, 19 Apr 2024 18:40:41 +0200