Fix tests breaking in GLib 2.59 (GHashTable order changed)

[robert-ancell]

No description provided.

[robert-ancell]

From #222

[robert-ancell]

This doesn't fix all the code that uses the ordering, but seems to be enough to fix the tests.

[ximion]

Looks good to me, thank you for doing all the work!
I also like that the sorting overhead ended up almost exclusively in the serialization functions (I don't think this will work forever though ^^).
The changes should be fairly complete, given that one of the tests involves loading a large XML file, serializing it to the GVariant cache, loading it and then comparing the result.
But any further issues will be uncovered by future testsuite expansions :-)