Move tests into subdirectory

This is a break out issue from #364467: Remove-items followed by update items does not work, see comments #35 to #37.

Problem:

Some modules have more than one test file. For instance if a module has a test module, it has at least three files: mymodule.test, mymodule_test.module, mymodule_test.info. This adds to the overall clutter in the modules directory, or 'filesoup' like webchick called it :).

Suggested solution:

Keep all test files in a subdirectory tests/ in the module directory.