[441483] Update SiriusTestsPlugin.PLUGIN_ID according to plugin rename

Bug: 441483
Change-Id: Iab3a058575e16589416413eea3e1c1c34d620710
Signed-off-by: Esteban Dugueperoux <esteban.dugueperoux@obeo.fr>
[441483] Publish the tests repository separately

This will avoid "polluting" the main repository with test-related
plug-ins and dependencies, that end-users could install by mistake if
they "Check All" in the main repo. It will also enable more targeted
location override in "test-drive" style builds.

Bug: 441483
Change-Id: I3ace7019eacc035b59e06d4463455385b88ac9f3
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>