add ddns tests

[leshow]

• adds minimal ddns tests
• fixes an issue with the multiping test
• remove sudo -E from test runner which was creating an error output in cargo test

[coveralls]

coverage: 71.654% (+2.8%) from 68.824%
when pulling d3a1549 on ddns_tests
into 76d4782 on master.

[leshow]

apologies to anyone subscribed to notifications, was struggling with the permissions for test runners. You should be able to run cargo test locally now and in github actions without any errors, at least.