dissectors: add unit test suite

This adds a unit test suite that can be used to test dissector code, roughly modelled after the wmem tests. In order to access functions that shouldn't be exported in libraries, epan, dissectors and a few dependencies are linked as objects instead of libraries, which is why the intermediate epan_objects was added. My hope is that this approach will improve on some of the issues that were raised during the discussion around happy-shark and capture tests. For now I didn't put the single test in a separate file, but that can easily be done as more tests are added.

For examples of tests I would like to add in the future, see zidel_/wireshark@e6f4a741