tests: Fix some minor leaks in the unit tests

https://bugzilla.gnome.org/show_bug.cgi?id=737446
Add version macros for 2.44
GCredentials: Fix ABI break when adding NetBSD support

https://bugzilla.gnome.org/show_bug.cgi?id=728256
gdesktopappinfo: Use symbolic names in the code

We have #defines for the key file groups, so use them to avoid typos.

https://bugzilla.gnome.org/show_bug.cgi?id=736273
GDataSet: silently accept NULL/0 as keys

This used to be the behaviour before we made these functions
threadsafe; keep it that way.

https://bugzilla.gnome.org/show_bug.cgi?id=737741
GDataSet: Add more tests

These tests exercise the NULL key fix from the previous commit.
Updated Italian translation