tests: workaround conffile prompt in "ubuntu-advanage-tools"

The "upgrade-all-security" test will check that all security
updates work with unattended-upgrades. But they don't anymore,
the `ubuntu-advantage-tools` package will cause a conffile prompt:
```
2024-05-25T18:53:23.6261939Z (old) conffile prompt found for /etc/ubuntu-advantage/uaclient.conf (ubuntu-pro-client)
```

In reality it does not because of {pre,post}inst magic but
unattended-upgrades cannot detect this - if only this metadata was
declarative instead of encoded in the bash scripts :(

debian/tests/upgrade-all-security

@@ -45,6 +45,12 @@ fi
 chroot_exec "$chroot_dir" apt-get install -y xfce4 apparmor hello 2>&1
 chroot_exec "$chroot_dir" apt-mark auto hello
 
+# the "ubuntu-advantage-tools" will pull in "ubuntu-pro-tools" which has
+# a conffile prompt (well, it does not because of {pre,post}inst magic but
+# unattended-upgrades cannot detect this, oh if only this was declarative
+# instead of imperative :(
+chroot_exec "$chroot_dir" apt-get autoremove -y ubuntu-advantage-tools
+
 # build and install updated python-apt since the one in the snapshot has memory allocation issues
 upgrade_python_apt "$chroot_dir" "$distro"