testing: Add fake-ifconfig-lo to ensure ::1 is available

[awelzel]

The stub always produces output that includes a lo interface that has 127.0.0.1 and ::1. This helps to run the test in environments where the lo interface does not have ::1. We don't really care if the address/interface is real, it's for checking some other logic.

---

This is needed because currently CI tests on Cirrus fail with:

01:42.933] [#3] command.install-cluster-backend-zeromq-ipv6 ... failed
[10:01:42.933]   % 'bash /zeek/auxil/zeekctl/testing/.tmp/command.install-cluster-backend-zeromq-ipv6/install-cluster-backend-zeromq-ipv6.test' failed unexpectedly (exit code 1)
[10:01:42.933]   % cat .stderr
[10:01:42.933]   + config=/zeek/auxil/zeekctl/build/testing/test.38783/spool/installed-scripts-do-not-touch/auto/zeekctl-config.zeek
[10:01:42.933]   + installfile etc/node.cfg__cluster
[10:01:42.933]   + set +x
[10:01:42.933]   + sed -i s/localhost/::1/g /zeek/auxil/zeekctl/build/testing/test.38783/etc/node.cfg
[10:01:42.933]   + zeekctl install
[10:01:42.933]   Error: must run zeekctl on same machine as the manager node. The manager node has IP address ::1 and this machine has IP addresses: 127.0.0.1, 10.72.21.8
[10:01:42.933]   + cleanup
[10:01:42.933]   + set +x
[10:01:42.933]