Fix subplugin regression tests

[bakpaul]

This PR fixes the problem of running regression tests in plugins when their references are inside of the plugin and not Regression (e.g. BeamAdapter)

The idea is to have a stand alone *.regression.tests file that also includes the relative path of the reference folder. This is meant to be placed at the top of the reference file in a relative manner (see PR 4420 of sofa repository for example). This allows two things :

1. Having a stand alone file ease the process of finding the reference folder for any new comer
2. Having a stand alone file ease the process of automatizing the retrieval of new regression test along with their reference folder without the need of modifying anything in the CI script.