tests: Selectively unset D-Bus environment variables
tests: Selectively unset D-Bus environment variables

Don’t always unset all D-Bus environment variables when starting up or
shutting down an isolated bus, as to do so might trample on other
FolksTestDBus instances for other kinds of bus (e.g. session vs. system).

Instead, always unset a few common environment variables (like DISPLAY),
but only unset the DBUS_*_BUS_ADDRESS variable corresponding to the type
of bus being start up or shut down.

This is a tweaked version of the patch from
bug: https://bugzilla.gnome.org/show_bug.cgi?id=712148

pwithnall/folks,pwithnall/folks,pwithnall/folks,GNOME/folks,GNOME/folks,GNOME/folks