gio/tests/gdbus-peer: make this work on non-Linux unixes
gio/tests/gdbus-peer: make this work on non-Linux unixes

The test was using a socket in a temporary directory, but not actually
creating that temporary directory. This worked fine on Linux since it
actually ended up using an abstract socket instead, but failed on
unixes without abstract sockets.

https://bugzilla.gnome.org/show_bug.cgi?id=657517

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