🩹 Disable auto starting external Jack server

Jack client API defaults to try start an external process Jack server when no Jack server is found. This causes lots of grief to Jack client application maintainers because it rarely ends in a properly configured Jack server instance running. End-users ends up trying to kill this auto-started Jack server before having to start their own using systemd user unit, QjackCtl, Cadence or Studio Controls.

This also disable timecode plugin tests from unit tests suit so that packaging can be tested on CI without any Jack server running.

• Pass JackNoStartServer flag to Jack client creation so Switcher never try to auto-start an external Jack server
• Fix JackClient constructor validation to exit if no Jack server is found
• Disable start Jack server from timecode plugin
• Fix LTCSource destructor validation to close jack_client
• Disable timecode plugin tests (fix #157 (closed))