contributing: mandate end-to-end tests, allow others

Update the section on adding tests, to fulfill these broad points:

- Mandate end-to-end testing, with rationale.
- Mention internal APIs as endpoints for testing.
- Warn against pitfalls of unit-testing.

It's more text than I would have liked, perhaps in later work we'll be
able to say it with less.

This change integrates feedback from the mailing list
'Guidance on Unit Tests' thread:
https://mail.gnome.org/archives/buildstream-list/2018-November/msg00045.html