FolksTestDBus: flush and close connection before disposing of it

g_dbus_connection_dispose() sets connection->worker = NULL, which
means any attempt to send a message will crash with a segfault.
If anything still has a ref to the connection - in Folks, that's
often a backend reffed by the IndividualAggregator - then it'll
tend to assume that it's still usable.

Attempts to send messages are guarded by a check whether the
connection is closed, which we can use to avoid this.

https://bugzilla.gnome.org/show_bug.cgi?id=726973
Make warnings non-fatal during "final teardown"

Historically, the Folks tests have not been expected to cope gracefully
with dropping off the session bus. This frequently results in warnings,
which default to being fatal. For instance, enable-disable-stores
fails with this fatal warning:

folks-WARNING **: Error preparing persona store 'eds:test': Couldn't get view for address book ‘test’: The connection is closed

The long-term solution would be to make sure that disconnection from
the bus is an identifiable error domain/code that gets propagated
through the stack, but that would have to be done individually
per-backend.

https://bugzilla.gnome.org/show_bug.cgi?id=726973
tests: avoid using dconf

tests/eds/perf reliably fails for me with this warning:

dconf-WARNING **: failed to commit changes to dconf: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name ca.desrt.dconf was not provided by any .service files

and in any case we shouldn't be changing the user's real configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=726973