Reviewed "Automated tests"

Related to #22. Depends on #34 too, merge #34 before this one.

The guide was still talking about:

- unit tests
- integration tests
- ui tests

I've renamed "integration tests" with "system tests" (as that's what I understood) and added a new "integration tests" which is basically old unit tests that used the database or the server.

Also just to be sure, I've seen plugins use `tests/` and `Test/` as a subdirectory. I don't know which one is the best, but I've edited the guide to be consistent (I've used `Test/` as I think that's the one I've seen the most in plugins, and that's the one which makes sense with autoloading).
`Test/Fixture` directory  --> `Test/Fixtures` directory 

(I note that Custom TrackerJs plugin uses Fixture/ but it's the only plugin so maybe we could change it to `Fixtures` and make the doc document `Fixtures` only?