tests: clear out various D-Bus-related variables

tests: clear out various D-Bus-related variables

The actual terminal in gnome-terminal 3.8 is an activatable service,
so it has DBUS_STARTER_ADDRESS in its environment. telepathy-glib prefers
DBUS_STARTER_ADDRESS over DBUS_SESSION_BUS_ADDRESS, but GTestDBus
doesn't clear or set the former, only the latter; so we end up trying to
use the real bus when run under a recent gnome-terminal.

See also <https://bugs.freedesktop.org/show_bug.cgi?id=63119>.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=697261
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>
[added FIXME comment about #697348]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>