LibsocialwebTest: don't try to keep ownership of name through tear_down()

LibsocialwebTest: don't try to keep ownership of name through tear_down()

This is a step towards replacing with-session-bus*.sh with GTestDBus,
which cleans up our D-Bus connection while the test is still running.
If we do that without this change, then the NameLost callback is triggered
when we get disconnected from D-Bus, causing an assertion failure.

This also should reduce circular references, by making sure closures
that contain "this" get cleaned up.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=690830
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>