add integration test for aid to join multisig

[daviddm]

This PR includes multisig test join and illustrates the problem in https://github.com/WebOfTrust/signify-ts/issues/227.

The integration test creates a multisig-group with 2 members and threshold 1. Member 1 adds a new member(member3) to the group and does a double rotation for the new member to be able to join and add the end-role by himself.