contributing: more clarity on testing
Description
Integrate feedback on testing from the mailing list 'Guidance on Unit Tests' thread: https://mail.gnome.org/archives/buildstream-list/2018-November/msg00045.html
Changes proposed in this merge request:
- "You must test changes in an end-to-end fashion" + explanation.
- Mention internal APIs as endpoints for testing.
- Warn against pitfalls of unit-testing.
Edited by Angelos Evripiotis