Tests: add session_bus_run() and use it where possible

This is to avoid having again the subtil bug in dbus-appinfo.c:
session_bus_down() was called before g_test_run() so the test was
running on the user's dbus session.

https://bugzilla.gnome.org/show_bug.cgi?id=697348
GTestDBus: Make sure only DBUS_SESSION_BUS_ADDRESS is set by default

g_test_dbus_unset() now also unset DBUS_STARTER_ADDRESS and
DBUS_STARTER_BUS_TYPE.

https://bugzilla.gnome.org/show_bug.cgi?id=697348
Tests: It is useless to unset "DBUS_SESSION_BUS_ADDRESS" manually

If the goal is to make sure we don't have a dbus connection, it has
to call g_test_dbus_unset() instead which is much more complete.

In this case, g_test_dbus_unset() is called already, so it should be
fine.

https://bugzilla.gnome.org/show_bug.cgi?id=697348