Build system API change, central definition of `test` target

Dear RIOT developers,

the way to define tests for an application has been changed. See the pull request for reference:

The `test` target is now defined in the main `Makefile.include`[1] and executes files from the `TESTS` variable. By default it is defined by listing executable files in the `application_dir/tests/` directory[2].

Please update your pull requests accordingly and remember to check this when reviewing other pull requests.

Thank you.

1: 2:

Cheers, Gaƫtan - @cladmi