Run idle-test and add more DBus manager tests

This is on top of !810 (merged)

It moves the idle-test from a -mslow test that needed to be run manually and with a running shell to an integration test that always runs against a freshly spawned compositor and the new libphosh from !805 (merged). This can be reused for further DBus interface tests in the future.

While at that we move all generated DBus sources to dbus/ to get cleaner test coverage.

This bumps us up to 47% (+10%) in CI and >50% on a 'real system' where all DBus services are running.