Merge: nitunit: testsuites are explicit

nitunit do no more try to be clever and find test-suites.

Previously, `nitunit foo.nit` caused the execution of the test suite `test_foo.nit` if it exist.
This behavior had some drawbacks:

* the test suite must follow a strict naming convention `test_*`
* the test suite must be in the same directory
* there must a an associated module. no way to create a test-suite out of thin air
* nitunit do nothing if you give the test suite instead of the module
* `-t` used to precise the location of the test suite did not work with more than one module

The proposed solution is to remove all this test-suite association with a module and asks the used to use the test-suite instead.

    nitunit test_foo.nit

Because nitunit in intended to work with more than one module, there is basically no change for client that do

    nitunit .

Pull-Request: #2167
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>