gtestdbus: Print the dbus address on a specific FD intead of stdout

We used to use a pipe for the dbus daemon stdout to read the defined
address, but that was already requiring a workaround to ensure that dbus
daemon children were then able to write to stdout.
However the current implementation is still causing troubles in some
cases in which the daemon is very verbose, leading to hangs when writing
to stdout.

As per this, just don't handle stdout ourself, but use instead a
specific pipe to get the address address. That can now be safely closed
once we've received the data we need.

This reverts commit d80adeaa.

Fixes: #2537
10 jobs for dbus-daemon-print-address-fd in 24 minutes and 21 seconds (queued for 5 seconds)