Tests for DNS Link functions

[jcoglan]

This is James from @neighbourhoodie, I have been working on the STF milestone M1 to improve test coverage, focussing on resolved. We have written a lot of tests for this module and we are splitting the work into a number of PRs, one for each test file we've added. Each new test file usually corresponds to a single source file, and exercises the functions in that source file directly.

This PR includes tests for resolved-link.c, providing coverage of functions relating to Link objects.