Update testing-overview.txt

- update for actual tests directory structure used
- describe how to use tox to run only a subset of tests
- add hint on how to run tests when debuggers are used
- also remove non-existent heat/testing/README.rst from MANINFEST.in

Change-Id: I1441f79a006a4dfba2d7ee54ca6f4f7577205c0d
Merge "Update testing-overview.txt"