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 (closed)

/cc @bberg