introduce cross-repo standardized source tests

Benjamin Schubertrequested to merge
bschubert/standardized-tests into master

Description

For context: https://mail.gnome.org/archives/buildstream-list/2019-December/msg00014.html

This adds a way of registering plugins to run a set of standardized tests on them.

Plugins are tested both against their master version (non failing) and last published version (failing test).

Edited by Benjamin Schubert

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