remove hook_simpletest, use tests directory in module dir.

Currently if you want to distribute tests with your module you have to write addition code in your module, which is completely superfluous if you are not running tests. This patch removes hook_simpletest completely and looks in the /tests/ folder of each module directory.

I think it is a nice simplification simply because it will remove 5 repeated lines of code in every module that wants to implement simpletests. It will also create a standard location where people know to look for/place tests.