Support Out of Tree tests

Headline:
Test developers can now keep their test definitions stored outside of the lava-test source tree, and use them by pointing lava-test to the url. This way, tests can be updated and modified without having to update the code inside lava-test.

Currently Abrek only support tests build inside the Abrek source code. We need to add support for adding external tests, like Abrek tests inside current “working set” of active python packages. We also need to add support for declarative tests which cannot contain any python code and is completely encapsulated in .json file

User stories:
As a test developer, I want able to extent the test scope myself and to distribute my own tests, following the lava-test standard, as a python package. I then able to run the tests with lava-tools so that I will increase quality of my boards.

As a test developer, I want to extent the test scope myself by register the json files containing the test instructions. I then able to run the tests with lava-tools so that I will increase quality of my boards.

Caching

Acceptance criteria:
It is possible to run a test in lava, that is not checked into the lava-test branch but rather pulled from somewhere else.