Added developer-notes for debugging unit tests with GDB

Summary:
While using GDB to debug tests is straight forward, it's not immediately obvious that it can be done.
These notes should help developers debug their tests in a more methodical manner.

Test Plan: I do this to debug tests and it works. :D

Reviewers: schancel, deadalnix, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: matiu, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1224