Fix test discovery for api and functional paths

Fix test discovery for api and functional paths

The use of the builtin unittest test loader was silently dropping tests
that couldn't be imported.

This change also drops the retargetable path from discovery in the api
path due to a previously-masked configuration problem, and fixes an
invalid import in a functional testing fixture module.

Fullstack tests are also disabled temporarily pending a fix for #1446261.

Change-Id: Ie44e45c117bd864538e7919dfcf499091fde7752
Related-Bug: #1440834
Related-Bug: #1443480
Closes-Bug: #1446405