New MLS test tool

[pcapriotti]

Adapt wire-server integration tests to new MLS testing tool (https://github.com/wireapp/mls-test-cli).

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If this PR changes development workflow or dependencies, they have been A) automated and B) documented under docs/developer/. All efforts have been taken to minimize development setup breakage or slowdown for co-workers.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[mdimjasevic]

Make sure to add a changelog.