Change the way tests run for Postorius.

Currently, each of the tests run using the MailmanClient from git head instead of the released version. So git-head of Postorius might become incompatible with the released version of MailmanClient and would never get tested.

Ideally, Postorius should only depend on the released version of MailmanClient, however, it would be good to test if the changes in MailmanClient's master branch affect Postorius in future, which would help us fix the issue before another version of MailmanClient is released. Ideally, this should be tested in Client, but I don't have any good ideas about that right now.