dep8 tests do not account for resolv.conf managed by systemd-resolved

the 'tgt-boot-test' grabs a cloud image, stuffs the updates inside,
serves that image out via tgt and then boots a kvm with overlay root
pointed at it.

It then executes code on the inside to verify that the expected dns
servers are in place. That test is currently failing (see below) because
the expected name servers are now not in /etc/resolv.conf, but could be
seen in output of 'systemd-resolve --status'.

FAIL: test_tgt_boot (__main__.CloudImageTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopkgtest.4kalKO/build.thd/open-iscsi-2.0.874/debian/tests/test-open-iscsi.py", line 259, in test_tgt_boot
    (dns_addr, resolvconf_contents)))
AssertionError: 10.1.1.4 not in resolvconf contents:
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.
nameserver 127.0.0.53

Related bugs:
 * bug 1713803: replacement of resolvconf with systemd needs integration