CONTRIBUTING.md: Update information about tests

Contributors are now supposed to run tests in the GitLab instance, so
this also needs to be reflected in the CONTRIBUTING.md as otherwise
people not used to forge workflows could completely miss that
information.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

CONTRIBUTING.md

@@ -4,8 +4,14 @@ The main repository for mat2 is on [0xacab]( https://0xacab.org/jvoisin/mat2 ),
 but you can send patches to jvoisin by [email](https://dustri.org/) if you prefer.
 
 Do feel free to pick up [an issue]( https://0xacab.org/jvoisin/mat2/issues )
-and to send a pull-request. Please do check that everything is fine by running the
-testsuite with `python3 -m unittest discover -v` before submitting one :)
+and to send a pull-request.
+
+Before sending the pull-request, please do check that everything is fine by
+running the full test suite in GitLab. To do that, after forking mat2 in GitLab,
+you need to go in Settings -> CI/CD -> Runner and there enable shared runners.
+
+Mat2 also has unit tests (that are also run in the full test suite). You can run
+them with `python3 -m unittest discover -v`.
 
 If you're fixing a bug or adding a new feature, please add tests accordingly,
 this will greatly improve the odds of your merge-request getting merged.