Fix tests setup.
Two issues made the test experience worse :

• The error message for the setup check for enough people in the workspace indicated the wrong amount of members. Having a lower number made a test fail.
• One member was deleted by the test. This is annoying since we can only create membership by creating invitations and manually getting the link in the invitation email. Creating a membership for oneself is simple since one can get the mail, but having to ask someone else to click the link is less simple. This MR mitigates this by allowing to set the membership that will be deleted in a GG_EMAIL env var.