tests: Don’t close stdout for dbus-daemon in GTestDBus

The stdout FD passed to dbus-daemon is propagated to all its child
processes, such as service activated processes. If we close the FD after
reading the bus address from the daemon, any child process which
subsequently writes to stdout (e.g. for logging) will get a SIGPIPE and
explode.

Instead of closing the stdout FD immediately after dbus-daemon has
spawned, keep it open until the daemon is killed.

This fixes the Tracker tests, as tracker-store writes startup
information to stdout.