Moved src/test/README to doc

Summary: Completes T358: This doc file is not easy to find with grep, as it does not reside in the doc directory with most other docs. This change fixes that and updates developer-notes and unit-tests to match.

Test Plan: read it

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1496