allow to auto-start dummy backend (needed for tests)
fix audio-engine dtor.

backends - once instantiated - keep a reference to the engine.
when audioengine is destroyed, the backends must be deinstantiated.
This fixes various unit-test cases.
Tests: ensure that libardour is initialized
allow spaces in TOP, cd to gtk2_ardour to avoid dup midnam, etc warnings
fix MTDM test
fix audioengine use in unit-tests.