tests/dbus-runner: Don't log to a loose ended pipe

We created pipes for the stdout of the spawned mock services. This resulted in the pipe being filled if enough things were logged, as nothing was reading from it. Change this to allow for two modes: verbose - where output is logged to the parent stderr, as well as non-verbose (default) - where things are logged directly to /dev/null.

This fixes frozen tests when running with --repeat and a high enough repeat count.