SubscriptionManager#create(boolean) currently creates a sub. with only one user, the owner.

iirc - an earlier version was also adding test.user.2 as an admin. This seems gone.

To facilitate the process of adding a user within a Hooks class of a consumer module, we should implement 

SaasTestsuiteInitializer#addUser(Subscription, userRole)
SaasTestsuiteInitializer#addAdminUser

Note: I have changed it from #addUser(Subscription, userRole) to #addAdminUser.
atm, the others roles cannot be enrolled, because we lack other users so far.

In a next step we also will need to add the editable.test.user - but this one is not yet created - though hoping to get it soon with SRE-3593 (after we were discussing the topic with Ruben)